How do i make a gui that changes an NPC's appearance?

Use Humanoid:ApplyDescription and Players:GetHumanoidDescriptionFromUserId

https://developer.roblox.com/en-us/api-reference/function/Humanoid/ApplyDescription https://developer.roblox.com/en-us/api-reference/function/Players/GetHumanoidDescriptionFromUserId

Use remote events as well

local script in textbox:

local textbox = -- your textbox
local event = -- your event

textbox.FocusLost:Connect(function() -- you put however you want to detect
	local id = tonumber(textbox.Text) -- just incase it stays a string
	event:FireServer(id)
end)

normal script in workspace or serverscriptservice:

local event - -- your event
local npc = -- your npc
local humanoid = npc:WaitForChild("Humanoid")
event.OnServerEvent:Connect(function(id)
	local description = game.Players:GetHumanoidDescriptionFromUserId(id)
	humanoid:ApplyDescription(description)
end)

hope this helps you