Hi Luis,
By default, HFSS sets the firs port(here I assume the only port) with 1 (W or V) amplitude and 0 phase. If it is Driven Modal, the power is defined for the port and the levels are in Watts. If it is Driven Terminal, the voltage at the port is defined and you see it is given in Volts. For Driven Terminal, you can choose whether incident voltage or total voltage you want to be set. (And if matching is good, they will be very close).
User can change the excitations levels. It is post processing, meaning that you don't need to run the simulation again. HFSS just updates the field reports accordingly. To do so, you can righ click on the Excitation (in the Project Manager) and go to Edit Sources.... You will see a list of all excitations used in the design with some default values and you can change it as desired.
You can read about Edit Sources in HFSS Help document.
You are navigating away from the AIS Discovery experience