I need to change check format.

In QBO, the option to change a check format or edit a check template is unavailable. 

If you want to add information on the paycheck, add it in the Memo field to print it on the check. Also, the available check types are Voucher and Standard.

Moreover, in case you want to purchase pre-printed checks from us, which includes this information. You can visit this website to open the Intuit Marketplace site: QuickBooks Checks & Supplies.

I'm leaving this article for future reference in setting up your print settings so you can print checks in QBO: Configure your print settings for printing checks in QuickBooks Online.

Please post again if you have follow-up queries about handling checks in QuickBooks. I'll be around to help you.