If you have never edited a Wikipedia page before, here is how to do it in a nutshell:
- Click on the tab at the top of the page that says Edit this page, or if you want to edit a specific section, click on the label to the right of the section title that says Edit.
- Add text in the edit window that appears, or else modify the existing text that you see there.
- When you are ready, click the button underneath the edit window that says Show preview. This will show you the result of your changes. If you want to make more changes, continue to edit in the edit window.
- When you are satisfied, write a brief description of your edit in the Edit summary area directly under the edit window.
- Click the button under the edit window that says Save page.
If you understand those instructions and feel comfortable with them, you can simply go ahead and edit. If you would like more information, the instructions are explained a bit more thoroughly below.
The "Edit this page" tab edit
Most Wikipedia pages can be edited by anybody. Some pages are "protected", though, either because they are very important or because they are frequent targets of abusive editing. If you see a tab saying Edit this page, you will be able to edit. If you don't have the necessary privileges to edit a given page, you won't see that tab. Instead you might see a tab saying View source or something similar. If you feel that an article needs editing and you can't do it, try opening a discussion of the issue on the associated talk page.
The Edit window edit
When you are editing a page, you will see an edit window — a rectangular area filled with text that you can modify. If you click inside it, you can set the position of the text cursor, and anything you type will appear there. Many simple changes can be made simply by adding or removing text. There are also a wide range of fancier things you can do — see Help:Editing for an introduction to some of them.
The "Show preview" button edit
Press this button (underneath the edit window) to see the result of your changes. It is a very good idea to get in the habit of doing this for every edit you make, before saving it. Not only will you spot a lot of typos and other simple errors, but also, as your editing gets more sophisticated, you will pick up errors that can totally ruin the appearance of a page.
The "Edit summary" area edit
You should make a habit of writing an edit summary for every edit you make, even if it is something as simple as "add info" for "fix typo". When other editors review your edit, they will approach it with a much more positive attitude if the edit has an edit summary. Editors who systematically neglect to use edit summaries are widely seen as bad editors. Everybody forgets occasionally, but good editors don't forget very often.
The "Save page" button edit
When you are satisfied with your changes and have written an edit summary, you can click the Save page button underneath the edit window. Doing so causes the edit to be saved on Wikipedia's servers, and in most cases it will mean that readers will immediately begin to see the article with your alterations.
There are a couple of exceptions. First, some sensitive articles have Pending changes protection, which means that some trusted editor will need to approve your edit before it becomes visible to the public. You don't need to do anything to make that happen, but it does mean that there will be a delay (usually short) before you see your changes — or if your edit is perceived as harmful, you might not see them at all. If that happens, you can click on the history tab at the top of the page to see a summary of the editorial actions that have occurred, and you can open a discussion on the talk page.
The other exception is that if some other editor modifies the page between the time you click Edit this page and the time you click Save page, you might get an "edit conflict", which means that it is technically impossible to save your edit without losing the other edit. This situation is unfortunately a bit tricky to deal with, so we won't try to explain it here — see Help:Edit conflict for more information.
Notes edit
Nothing that you do is visible to readers or other editors until you click the Save page button. If you change your mind about an edit and decide that you would like to abandon it, all you have to do is navigate elsewhere in your browser. If you never hit Save page, nobody will ever know what you have been doing.
If you have been editing in a state of high emotion (as sometimes happens), it is a good idea to think hard about whether you really want to save your changes. An angry message is harmless if nobody ever sees it, but once you have hit Save page, it is nearly impossible to make your edit invisible.
After a page has been altered, there is a high probability that other editors will review the edit, often within seconds or minutes. (There are many editors who "watch" pages that they are interested in, or scan a special list of "recent changes".) If your edit is perceived as violating Wikipedia's policies, it may quickly be reverted, i.e., undone. If that happens, please don't panic or get angry, just look at the page history to see if any explanation was given for the revert, and if you need more information, open a discussion on the talk page. Even the best and most experienced editors occasionally have some of their edits reverted.
Happy editing!