Amending an electronic self assessment return

I submitted my 22/23 self assessment return via a commercial software supplier and now need to amend this. However, the supplier's software does not offer the opportunity to amend a return once submitted. How can I amend this return? Thanks.

Hi LostinGuernsey,
You will now need to send the amendment in writing stating the relevant boxes and figures.
Send this to
HMRC PAYE & Self Assessment
BX9 1AS
Thank you.