I'm having issues with editing spreadsheet cells. If I try to modify existing text or numbers in the top bar as usual, I can't--it just types my new text as if the cell is starting off empty. 

In other words, let's say my cell read

dollrs

and I want to add an 'a'. I click where I want to put it and type 'a'. Instead of 'dollars', I just have

a

Sometimes I can undo and at least save the existing text so I can modify it elsewhere, but much of the time, I end up having to retype the entire cell.

Can you help? Thanks.