Thank you very much for your help MDQ, since I am not an expert in heating I wanted to confirm what you recommended in case of not finding the drain valve:
1) Heating water outlet key closure
2) Disconnect return going to the boiler, I put a hose and the grill.
3) I opened a key that had closed the water outlet of the water heater
4) I open boiler filling wrench
Does the boiler have to be on or off?
Thank you very much again for your help