I issued checks to a vendor which were then lost in the mail. I will reissue the checks and place a stop payment with our bank. How do I correctly remove them from QBO?

Welcome, @khart4. 

Congrats on making your first post here in the Community. I'd be more than happy to help you remove these checks from your QuickBooks Online account. 

It only takes a few easy steps to void your checks that were issued to your vendor. Here's how: 

1. Go to the Expenses tab on the left-hand menu bar. 
2. Ensure that you're on the Expenses portion of the page. 
3. Scroll down and locate one of the checks you need to void. 
4. Press the drop-down arrow in the "Action" column. 
5. Choose Void. 

You can do the same instructions for the other check. 

This should do the trick. I'm only a comment away if you need me. Have a wonderful day!