This is typically relevant when exchanging SIMs from T-Mobile to AT&T.
Once logged in to your Dashboard, head to the Devices page and click on the relevant device.
In the top right corner, you'll see a Settings button. Click on this to access the Device Settings page.
Click on the Manage SIMs option and then enter the ICCID of the SIM you want to add to this device. When you're done, click on 'Update SIM' and you're all set!