How do you switch items when you equip another?

I will tell you what you need the script to do.

you have to make it so that when something is added to the LocalPlayer's backpack, it will call a function. (which means you have to use a local script)

then inside the function, you have to get the children of the backpack and remove all items, then re add the item that the player got into the backpack.

Here's a local script that may help (make sure to put in StarterPlayerScripts):

game.Players.LocalPlayer:WaitForChild("Backpack") --waits for the backpack to load (just in case)

--Variables:
local plr = game.Players.LocalPlayer 
local bp = plr.Backpack

bp.ChildAdded:Connect(function(child) --calls a function when a child is added to the backpack
    if v:IsA("Tool") then --checks if the item added is a Tool
        local Key = child:Clone() --Clones the added child because we will destroy everything in the backpack
        for _,v in pairs(bp:GetChildren()) do --gets every children of the backpack
            v:Destroy() --destroys them all
        end
        Key.Parent = bp --readds the child to the backpack
    end
end)