Hi there, @JI1.
Welcome to the Community. I'd be glad to assist you with entering a check number in numerical order into QuickBooks Online.
QuickBooks automatically increment the check numbers unless if you turn on the Custom Transaction number feature. Therefore, once you assign one check number, the rest will regularly follow in order.
Let's go ahead and make sure that the Custom Transaction number is not enabled.
Once verified, you can edit those checks that you've created, then enter your desired check number.
I've included an article about changing transaction numbers on sales forms for your future reference: How to change the invoice number on sales forms.
I'm confident that these resources will assist you with entering the check numbers in numerical order into QuickBooks.
Don't hesitate to let me know if I can be of additional assistance. Have a wonderful weekend!
Use Banking menu > Write Check. Enter the Check. It doesn't matter when you enter it; you still put the needed info, including date, on it, whenever you enter it.