Then you should select Design/Configure Power Rails from the menu on the top and add a new power rail with a suitable name (e.g. +12V).
After you've added your new power net, go the hidden pins window i mentioned in my previous message and change the power net from VDD to +12V from there.
If you're using only a single supply which is different than 5V, you don't even need to do this. The fact that the voltage level of the VDD net is defined as 5v does not mean that you have to connect a 5V supply to it.