The official answer provided is only partially correct.  Here's the rest of it:

The ONLY effective guide on this can be found here:  https://quickbooks.intuit.com/learn-support/en-us/import-or-export-data-files/export-and-convert-you...

 Things to keep in mind:

• This will almost always end up as an escalated ticket with Data Services because this process has always resulted in a corrupted file every time I've ever used it.
• Turn off your computer's Anti Virus while using the migration tool
• When you open your local QB Desktop program while using the migration tool, be sure to open it as administrator.  It makes a difference.
• When the migration tool has completed and you have the resulting file open, rune a rebuild immediately; it will show errors, so you might as well get that out of the way before support asks you about it.
• Compare your chart of accounts and verify all of the balances for each line item is correct.  Note the ones that are not correct.
• Open payroll and compare it with your online company file.  You may have extra employees or are missing employees.
• Contact QB support and request a data services ticket and list all of the problems that you have found.  Ask for a manager after the 2nd-or 3rd time they ask you something you've already told them.
• 50% of the time you will be hung up on; just try it again and immediately ask for a manager and repeat your request for a Data Service ticket.
• If you have done all of the above, have double-checked that you followed every step in the guide Intuit wrote for this in the link above and you provide them with a list or each problem that you found with the new file, you will spend considerably less time on this and greatly reduce your own frustration.
• If you get a bad tech and they begin to argue with you:  Don't waste your time.  Hang up or end the chat and start it again with someone else.  Reference the other attempt and move on with someone new.  each try gives you a chance of having a better tech on the line with you..
• At all time be firm, but polite and professional.  Intuit knows they have a lot of challenges with their support so there's no need to waste your energy or make their day aggravating if you don't have to.  It is what it is, so navigate the issues as outlined above and you will greatly reduce your stress.
• The ONLY way to provide support with your company ID is to log into QBO and while on the home page of your company, hit the combination of these keys:  CTRL-ALT-/  That open a new window with your company ID in it.  Support will always ask for that.
• If you do not want your file to end up in the 2022 version, because it's not all that stable yet and while my own company is using it, I will not put any of my clients in it due to a lot of update and other issues that still exist.  Make them aware of exactly which version you are using and that you expect the result to be in that same version or they will default to premier 2022.