Among the many user editable files in a standard WordPress installation are the Plugin files. Though it should be rare that you need to change a Plugin code, the Plugins Editor Screen allows you to edit those Plugin files.
Back to Administration Screens.
The built-in Plugins Editor allows you to view or change any Plugin PHP code in the large text (or edit) box that dominates this Screen.
If a particular file is writeable (see Changing File Permissions), you can make changes and save the file from here. If not, you will see the message You need to make this file writable before you can save your changes. See the Codex for more information.
The name of the Plugin file being edited shows up at the top of the text box. Since Plugin files are pure text, no images or pictures can be inserted into the text box.
You can select a Plugin to edit from the dropdown menu on the top right. Just find a Plugin name and click "Select."
Remember to click this button to save the changes you have made to the Plugin file. After clicking this button you should see a splash message at the top of the screen saying "File edited successfully." If you don't see that message, then your changes are not saved! Note that if a file is not writeable the Update File button will not be available.