How can i say how many watt should my power supply in my pc be?

Answer 1

Check the manual. It should tell you.
If not, get the make and model of ther power supply and do internet search. It will tell you if its a 350W, 400W, 450W etc.

Answer 2

I dont believe you can, unless you want to change the voltage (115v -220v) there is usually a switch on the PSU on the outside of the computer, but otherwise, the voltage is hard wired into the opwersupply, I think you mean going between Europen Power to American power, where there is a switch on the back.