To edit a field in the database, you must first be in Document View, on the Text View tab. It is also generally advisable to customize your view and promote fields you intend to edit to the top, so you do not have to scroll down for them.
Once you have your field order set, click on the Edit button, highlighted in yellow below.
A light box will appear around any fields that your login has permissions to edit. Click in the box to begin typing. Clicking on the Edit icon with the x cancels the edit, and the check saves it. (Note : if you move to another record without saving, the edit will be lost.)