QB Issue Resolution:

Sorry to say that in QBO, it isn’t possible to undo or edit a reconciliation that is already completed. As a work around, you can unmark all the reconciled transactions so that their status becomes unreconciled and therefore allowing you to re-do the previous month’s reconciliation. Here’s how you can do so..

1.) Click on the gear icon at the top right hand corner
2.) Select Chart of Accounts
3.) Click on Account History for the bank account concerned
4.) Transactions marked with an R means that they have been reconciled. Keep clicking on the R till it disappears and then save the transaction. This would render the transaction’s status as unreconciled thereby allowing you to perform the reconciliation again. Please repeat this for all transactions in the months that you wish to reconcile

PS : Previously reconciled bank reconciliation records would still remain in your Reconcile page. Therefore please make a note to yourself that some of them would no longer be relevant once you’ve carried out the above steps.

Resolution for Issue 'Reconcile error.After I reconciled Jan.2016, I made some changes.Now there are some error because of changed transactions.' available: Yes (Solved).
Source: Intuit Community forum.

Reconcile error.After I reconciled Jan.2016, I made some changes.Now there are some error because of changed transactions.: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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