If you submitted your return via ZAPTAX you will be able to see the date it was received by HMRC in your tax account on the hmrc.go.uk site, but you won't be able to amend your return there yet. You can either wait till the next update of ZAPTAX coming soon, when you'll be able to amend your return in your ZAPTAX account, or you can contact HMRC directly. You have twelve months from the tax deadline to make any amendments you need to. You can see more details here:

https://www.gov.uk/self-assessment-tax-returns/corrections