Once you have the motherboard out you should have something that looks a bit like this.

The CPU is the one that's mounted at the funky 45° angle. Bottom left on the overall photo.

On the side of the motherboard that normally faces the back case there's huge Phillips head Spring loaded screws holding the heatsink on, but on the other side the usual torx T10 back to back with the Phillips head things. Four to undo.

When you have that heatsink off you need to clean the heatsink side with some thermal cleaner. Fixit sell it.

The i7 processor is socketed and easy to unclip. There's a spring loaded handle top right orientation in the overall photo although you can't see it because of the heat sink. In the 4th image it's visible to the left of the socket.

The old processor lifts out and the new processor drops in be careful to align the processors identically. One corner has an arrow on it.

Use new thermal paste between the processor and the heatsink. IFixit sell it.