Pressing F2 will shift between Excel's editing modes. It's easy to tell which mode you're currently using by checking the bottom left corner of the Excel Window.
Generally, Enter
is the mode that will overwrite the current contents of the cell highlighted.
Generally, Edit
will place the cursor back in your cell to change the cell's internal contents.
And, generally, Point
is used when a dialog box is being used to select a cell or range (like selecting data for a chart).
EDIT: So, in answer to your question (finally), once your cell has a formula in it, you'll need to select F2 once to go into Edit
mode, add an operator (like +), then select F2 a second time to enter Enter
mode, then use your arrow keys and it automatically changes to Point
mode to select your cell.
EDIT: To modify an existing entry (without adding an additional operator), you need to use F2 to enter Edit
mode, then use your arrow keys to find the cell reference in your formula that you want to modify, then delete the old reference and press F2 again to enter Enter
mode, then you can use your arrow keys to select your new reference in place of your old.