How do I put a player's camera back to themselves after changing it?

You can reset the camera back to the character by doing this:

if game.Players.LocalPlayer.Character then
    cam.CameraSubject = game.Players.LocalPlayer.Character.Humanoid
end
cam.CameraType = Enum.CameraType.Custom

It first checks if the character exists. Then, it sets the camera subject to the corresponding humanoid. Finally, it sets the camera type to custom (this allows the player to choose their own camera mode). You may change this if you had a setting for the CameraType before.

the way you can do this is by first setting the camera subject and then the cameratype back to custom.

local cam = workspace.CurrentCamera 
local camPart = workspace:WaitForChild("Cam")

cam.CameraType = Enum.CameraType.Scriptable 
cam.CFrame = camPart.CFrame + cam.CFrame.lookVector * 2 
cam.CameraSubject = workspace:WaitForChild("Cam") 
camPart.Transparency = 1

delay(5,function()
    cam.CameraSubject = game.Players.LocalPlayer.Character.Humanoid
    cam.CameraType = Enum.CameraType.Custom
end)

So this basically did everything you did in the script above (please use codeblocks) and makes a coroutine 5 seconds after the

cam.CameraType = Enum.CameraType.Scriptable 
cam.CFrame = camPart.CFrame + cam.CFrame.lookVector * 2 
cam.CameraSubject = workspace:WaitForChild("Cam") 
camPart.Transparency = 1

runs. In the coroutine, it makes the player the camera subject and makes the camera type "custom"