How can I change weapons by scrolling?

To do this then you should follow these steps:

1) - Make a table full of all the tools you want the player to have access to

2) - Make a variable for how you'll index the table.

3) - Change the variable according to the WheelForward and WheelBackward Events

4) Replace the tools in the character with the WheelForward and WheelBackward Events, and index the tool table according to the variable.

Should look something like this;

NOTE: This should be a localscript in StarterGui

local tools = {} --Tool table
local plr = game.Players.LocalPlayer --Player
local mouse = plr:GetMouse() --Mouse
local toolIndex = 1 --Variable for indexing tools table

repeat wait() until plr.Character --Wait for character

local char = plr.Character

function clearTools(c) --Function to remove tools
    for i,v in pairs(c:GetChildren()) do
        if v:IsA('Tool') or v:IsA('HopperBin') then
            v:Destroy()
        end
    end
end

function replaceTool(c)
    local tool = tools[toolIndex]
    if tool then
        clearTools(c)
        tool:Clone().Parent = c
    end
end

mouse.WheelForward:connect(function()
    toolIndex = toolIndex + 1
    replaceTool(char)
end)

mouse.WheelBackward:connect(function()
    toolIndex = toolIndex - 1
    replaceTool(char)
end)