How to change invoice number from new tax year?

Apologies for missing the second part of the post!

You can lock a year to prevent modifications to the data. You can either do this by setting the lock manually, or running the year end tool. But this doesn’t remove the data from your dashboard.

There’s more information on the year end tool, here: Simplified year-end tool
With locking the account, you can find this option in Account Settings >> All Settings >> Account Lock Date