The 'F3' button located on the toolbar at the top of the page can be selected by using the corresponding F3 key on the keyboard or by using a mouse click.

F3 - Accept

When this button appears in the toolbar, you can select F3 to select into a record which will allow you to edit the record.

Example

The screen below contains grey-shaded fields, which signifies that it is read-only.  In order to be able to edit the fields you must select 'F3 - Accept' and the grey-shaded fields will become white.