Trying to Clean Up an Account

I moved to Quickbooks Online because it was integrated with a case management system that I switched over to.

The transition did not go smoothly.  The banking transactions now seem to be working as they should, but with the problems from before the account records are all messed up with all of the mis- and un-matched transactions that occurred prior to getting everything worked out, to the point that the Quickbooks Online balance and the actual bank balance are tens of thousands off.

Is there a (hopefully quick and easy) way to basically start over from the beginning of this year or part of last year to get everything in line?  I thought about deleting the entries from a prior point, such as 31 December and then just go from there -- but of course Quickbooks Online is linked to my bank so I did not know if that would work or if Quickbooks Online would just import the older transactions and get messed up anyway.

Help would be greatly appreciated!