Thanks for reaching out, @sindynanan.
Adjusting a memo on pre-printed checks is currently unavailable in QuickBooks Online. You can only adjust the alignment.
When printing, I suggest using a blank paper first. This way, you can easily correct alignment and to ensure it fits on the details.
Consider checking these articles for more information to set up Print checks and how to fix print alignment for preprinted checks in QBO:
Don’t hesitate to get back on our page in case you need help with QuickBooks. You can count on me anytime.