How to change a string value on leaderstats?

script.Parent.Touched:Connect(function(obj)
    local plr = game.Players:GetPlayerFromCharacter(obj.Parent)
    if plr then
        plr.leaderstats.Area.Value = "Noob"
    end
end)

You forgot to put ".Value" after plr.leaderstats.Area

The reason to why you're unable to change the value, is because you forgot to add .Value at line 3.

-- instead of 
plr.leaderstats.Area = "Noob"

-- change it to
plr.leaderstats.Area.Value = "Noob"

It's as simple as that

EDIT: To fix the error 'leaderstat is not a valid member of MeshPart' you have to set the leaderstats values through player and not the character. This is something that i completely forgot about.

script.Parent.Touched:Connect(function(plr)
    -- this line retrieves the player from the character
    local player = game.Players:GetPlayerFromCharacter(plr.Parent)

    if plr.Parent:FindFirstChild("Humanoid") then
        -- we change "plr" to our new "player" variable instead
        player.leaderstats.Area = "Noob"
    end
end)