Helloo,
I have stumbled upon this wiki:
https://developer.roblox.com/api-reference/class/StarterPlayer
There is a method to change everyone’s character to a custom one from the start.
However, my question is, how would you change a single character to a custom one upon pressing a key button?
Thanks
You’ll have to create this yourself, either using the HumanoidDescription system or by modifying the character model yourself. There’s no way to isolate StarterCharacter for a single person, nor is that what you seek to use for this kind of a system.
An old, and kind of hacky method, for doing this is to have a model (either pre-made in studio, or constructed by a script) which is a valid character (so has a humanoid, animation scripts, etc.).
When a player wishes to change their character, you clone that character and set the parent to the player. Next, you set the character property to that model inside the player.
Now, you can move the model, but you will notice that the camera is static. You can fix this by setting camera focus to the new character model.
IMPORTANT NOTE: Before anybody calls me out for using hacky methods and telling me the problems with this, I just wanted to say that I know. However, many games do actually use this method. I’ve seen many uncopylocked games which use this method and work perfectly.
Hope this helps you 
Hello. For the key pressed event, you can use UserInputService. After the key is pressed you will have to load the character with a HumanoidDescription. (There are other ways but I really recommend HumanoidDescription)
Here you will find more about HumanoidDescription.
Here you will find more about UserInputService.