How can I open this locked window?
I have opened a similar broken handle. Gently wiggle a thin piece of steel, e.g. stanley blade, between the rotating handle and the fixed base, below the black part of the button. There should be a sprung plastic catch that can be pushed up towards the button and then the handle should turn.

Check your other working lock first. When it is open you should be able to feel the catch.

There are usually two screws holding the handle in place. One under the gold cover on the left and one under the handle when it is open. This is a common window handle and should be easy to find replacements from online stores or local window fitters.