If you wish to start fresh and declare an account balanced, you can
mark all entries balanced. Select to show only unreconciled
transactions. Sort by date, and go to the oldest transaction.
Hold down Cntl+Shift+M and let auto-repeat work thru the
transactions until none are left showing. Do not change the
starting date or starting balance. The starting balance is the sum
of the transactions that have already been marked as reconciled.
Then balance making the ending balance match the known balance.
Choose the balance date to be the day after your last transaction.
This is not the normal procedure. It starts you with an amnesty
for former balancing.
Thereafter, follow the normal procedure.