Just as HMRC has the right to repair an obvious error or mistake on a return the taxpayer has the right to amend it, within 12 months of the filing date for the tax return, which in the majority of cases is the following 31st January.
The amendment to your tax return may be in the form of a letter detailing the omissions, an amended return, an extra supplementary page that shows the relevant source of income, or an amended supplementary page of one originally submitted with the return.
HMRC will normally accept an amendment to a return or self-assessment, whether it is notified by the taxpayer or by their agent. However, you must ensure the amendment is supplied in writing.
No penalties for late filing will be applicable, but you need to be aware that if the amendment results in additional tax to pay you will be charged interest from the due date of payment, which in your case was 31st January 2020, and may be liable to a 5% surcharge if this is not paid by the end of February 2020.