Change Player's Health

I don't know how to change the Player's Health in Roblox.

I tried this:

local Object = game.Workspace.Baseplate
Object.Touched:Connect(function()
     game.StarterPlayer.HealthDisplayDistance = 50
end)
local Killbrick = game.Workspace.Part

I thought the 3rd Line changed the health of the Player but it didn't do anything.

Your issue is that the Health property isn't on a Player, but rather their Character model's Humanoid.

-- set health to 50 when touching the baseplate
local bp = game.Workspace.Baseplate
bp.Touched:Connect(function(otherPart)
    -- double check that the other part is a character model
    local player = game.Players:GetPlayerFromCharacter(otherPart.Parent) 
    if player then
        -- set the player's health
        player.Character.Humanoid.Health = 50
    end
end)

-- make a kill brick
local Killbrick = game.Workspace.Part
Killbrick.Touched:Connect(function(otherPart.Parent)
    -- double check that the other part is a character model
    local player = game.Players:GetPlayerFromCharacter(otherPart) 
    if player then
        -- kill the player
        player.Character.Humanoid.Health = 0
    end
end)