You modify check specs under Maintenance | Maintain Specifications. You then open up the check spec you want to modify, and make the changes.
You can view a list of all of the spec codes by selecting View | Spec List. Then click on the plus key to the left of Company Information, which expands those codes, and drag "M18 Cash Invoice Number" to where you want it to print.
You can drag other fields around if you need to reposition them on the check. Whatever you do, don't adjust the heights of each section! That's asking for trouble.
Modifying check specs requires more than an elementary knowledge of Adagio. If this explanation is over your head, either watch the recorded webinar on Adagio Payables, or contact your Adagio consultant for help (or read the manual!).
Steve