Here are the steps:
Remove the strings to have a comfortable working space.
Access the switch: usually the switch is on the top of the guitar, you remove the outer screw of the old switch.
Pull the switch out: due to limited space inside, you need to gently pull the old switch out through the removed hole.
Tip: before pulling it out completely, tie a small piece of nylon string to the switch so that it is easier to thread the new switch back into the correct hole later.
Note the wires: take a picture or note how the wires are connected to the switch (bridge, neck, output, ground).
Unsolder: use a soldering iron to remove the wires from the old switch.
Install the new switch: solder the wires according to the old diagram (refer to the photo). Gretsch OEM switches usually have a 3-position diagram similar to Switchcraft, just attach the wires to the correct position.
Thread the switch back into the guitar: use the previously tied nylon/parachute string, pull the new switch through the hole on the guitar top and tighten the outer screw.