Hi, not sure what you mean by "linked the deposit to the bank invoice" but the straightforward easy way of correcting a cashbook entry that has been updated:
Go to the same cashbook / receipts where original wrong entry was made.
1. First entry line: Exactly the same as the "wrong" allocation entry - same tax code GL/customer code everything... same date and I prefer to always use exactly the same reference as the original wrong entry as well - AMOUNT entered with a MINUS in front
2. Second entry line: The correct entry with GL then 8100/000, same reference, same tax code etc and enter amount as usual (positive)
3. Always check that the batch total at right bottom now says "In balance" so two entries cancels each other out / effect on bank account is zero.

Update and done!

If you are not certain of the details of the incorrect allocation entry - go to View / General Ledger / Transactions / Entry Type Details - select the period and entry type e.g. CB 1 Receipts and I usually choose without generated entries so x must not be in block next to Show Generated Entries - scroll until you find your wrong entry.
But it seems like you already know the details and have seen the wrong entry in the ledger.

Good luck! :-)