I'll take that you want to change the license number on your QuickBooks Desktop for Mac.
Let me guide you with these steps:
From the QuickBooks menu, select Manage My License.
Select Deauthorize This Computer.
Click Deauthorize and Quit.
From there, re-open QuickBooks and follow the registration process. It'll let you enter a different license.
On the other hand, QuickBooks Online doesn't have a license number. They have a Company ID. You can locate it on the Billing & Subscription page. Here's how: