My accountant has asked me to put an adjustment in to 2019/2020 year to bring bank acc balances back in line. can you advise how I can do this?

Welcome to the Community, @info1231.

You're referring to adjusting your opening balance in QuickBooks Online (QBO), correct? If so, the steps do these are pretty quick and easy. All you need to do is to go to Account history.

Here's how:

1. In QBO, go to the Accounting menu at the left pane to get to the Chart of Accounts.
2. Locate the account, then tick Account history under the Action column. 
3. Find and select the opening balance entry, then click Edit.
4. Enter the necessary changes.
5. Hit Save to complete.

For more information about the process, refer to this article: Edit an incorrect opening balance [Video].

Just in case, I'll add this article for future reference: How do I enter a beginning balance for an account after it has been set up?

Please know that I'm only a few clicks away if you have any other questions or concerns. I'll be here to assist. Have a wonderful day!