The latch is in the first photo. It's locked in the photo, turn it clockwise so it's upright.

Like people have said, it's probably stuck from paint or caulk or dirt. If there is a screen or storm window on the outside, it probably latches from the inside so you can't go outside and push at the bottom, but if you can , do that. Maybe use a screwdriver to pry the window to get it to move a little and work it loose.

The window swings up, once the window opens the top hinges get somewhat loose so you can pull the window back towards you a little bit. Once it opens up 90 degrees, you can push it back in the little bit you pulled it out and the 'tabs' on the window part of the hinge will then fit into the 'hooks' on the frame part of the hinge and hold the window open.

When the window is up (open), I believe you can jockey it around a little, lifting one side or the other a little bit and the window will fairly easily come right out of the frame with the window part of the hinges still attached. The window can then be turned over upside down, and the hinges still attached to the window will fit into those hinges sticking out at the bottom of the frame. The 'tab' and 'hook' arrangement will then work to hold the window open with the top leaning in around 45 degrees. That's so you can leave it open long term for ventilation and it will tend to keep out the majority of the rain.