Before touching a single transaction, back up the company file and confirm the cleanup start date with whoever owns the books. In QuickBooks Desktop that is File → Back Up Company → Create Local Backup, followed by File → Utilities → Verify Data: if Verify reports damage, run Rebuild Data (it makes its own backup first) or stop and get file repair before any bookkeeping, because corrections made inside a damaged file can vanish. QuickBooks Online keeps no restorable copy you can roll back to, so save the profit and loss, balance sheet and each account's last reconciliation report as PDFs before you start. Everything before that date is history you reconcile; everything after is what you'll keep current.
Skipping this step is how cleanups turn into do-overs. Establishing that point means three concrete checks. First, find the last month every account reconciled with a zero difference — that is your true starting line, whatever the calendar says. Second, gather the bank and credit-card statements for every account from that month forward, in one place, before you open a single register; reconciliation stalls the moment a statement is missing, so collect them all now. Third, note the last tax year that was filed. Periods already reported to the IRS shouldn't be silently rewritten — corrections that touch a filed year belong in entries your CPA can see and explain, so flag them as you go rather than changing history without a trail.