Google Spreadsheet forces you to be in overtype mode whenever you edit an existing formula.
The only way out I know is to type gibberish. That is, for how every many characters you want to insert, you type however many gibberish -- while still losing a few meaningful formula characters.
Is there a way convert to insert mode when editing formulas on Google Spreadsheets?
Note: Insert key does not work
Insertkey on your keyboard hasn't been pressed? – Michael Frank Nov 12 '15 at 22:47