How to compress valve springs?

If the head is still on the engine, you can purchase a kit at most auto parts stores that includes: an air fitting to keep the cylinder filled with compressed air to prevent the valve from dropping into the cylinder, and a special wrench to attach to the rocker stud and press down on the spring retainer while you remove the locks.

If the head is off the engine, you will need a valve spring compressor, which is a large c-shaped tool that pushes on the valve's head on one end and the valve spring retainer on the other end.

Also you can buy/rent a valve spring compressor that you can use while the head is still on the engine but you will still need to blow air into the cylinder through the spark plug hole to keep the valve up.