QuickBooks Online's bank feed handles day-to-day syncing, but plenty of situations need a manual upload: history older than the feed covers, banks that won't connect, closed accounts, or a feed that quietly stopped syncing.
The upload flow
Go to Transactions → Bank transactions, open the account's dropdown, and choose Upload from file. QuickBooks accepts CSV and QBO/OFX files here.
CSV vs QBO: which to upload
With a CSV, QuickBooks makes you map columns by hand — and its mapper chokes on multi-column debit/credit layouts, thousands separators, and some date formats. A QBO file skips the mapping screen entirely because the format already encodes dates, amounts, and signs unambiguously. If your bank's CSV is anything but plain three-column, converting it with the CSV to QBO converter first is faster than fighting the mapper.
Sign conventions
Money out should be negative, money in positive. Credit-card exports from some banks (Amex and Discover in particular) flip this. If your upload shows purchases as deposits, convert the file with the "Flip signs" option ticked instead of re-mapping in QuickBooks.