Can't change self-assessment submission

Hi, I recently had to submit 3 years of self assessment forms for the first time. I made an error on the 2021 submission but there is no option to change it where it says their should be - I can only change the most recent 2023 submission. I tried calling but after holding for an hour was cut off by the HMRC person! Can anyone advise how best to do this? Thanks

Hi Matt,
It is now too late to amend the 20/21 tax return online, you will now need to write in with the details.
Contact details here:
SACM12150 - Overpayment relief: Form of claims
Thank you.