It’s not really “deleting” past months so much as shifting the view focus to new months.
One of the best things about personal finance in a spreadsheet is that you can personalize templates to your liking. If you unhide the columns off to the right, you will see a bunch of hidden data organization. You could try adding columns to the right of AU (in the latest build) to add reporting periods and then extend the formulas from AJ2:AJ to the right.
To extend the date range, you will probably need to fiddle with T7:T.
And finally, you will need to map these new periods into the visible portion of the template by adding columns to the right of O and extending the formulas from the left.
Let us know how it goes!