Hello Alex, you will need to migrate your company file from QuickBooks Desktop (QBDT) to QuickBooks Online (QBO).
First, you'll have to sign in as an admin to the desktop company file you want to move. Press Ctrl + 1, then Ctrl + B + Q to open the export window and select Get started. Log in to your QuickBooks Online company, choose the destination file you want to replace, and click Continue. If you track inventory, please confirm your preference and specify an 'as-of' date. Type Agree in the text field and click Replace to confirm the data transfer.
For a detailed step-by-step process, please visit this article: Move Your QuickBooks Desktop Company File to QuickBooks Online.
Once the migration is finished, run matching reports in both QuickBooks Online and Desktop to verify that your data is perfectly synced and organized. If you notice any discrepancies in your totals or accounts, consult the conversion guide to reconcile any platform-specific differences.
However, if you are only looking to bring in specific transactions rather than performing a full migration, you can click the Gear icon and select Import Data to upload your files directly within your QuickBooks Online account.

For additional questions or concerns, feel free to leave a comment below.