Hello,
Please login to RMS, then click on your email in top right corner of the screen and select "Credits -> Credit codes" option. In case you have created RMS credit code before, select it and choose option "Credit codes -> Remove code". That would return credits, which we used to create such credit code.
Once you would have remaining unused credits in your account (current balance can be seen by clicking on your email in top right corner), you will be able to navigate to "Services -> Management" RMS menu and click toggle button in "service" column to active credit for your device.