how do you make a currency conversion script?

hit would be the part that touched your script.Parent, you'd want to get the player, because that is where the leaderstats is stored.

The player service has a method to get the player from the character with internal validation. local Player = game.Players:GetPlayerFromCharacter(Character)

if a character is provided, it will return the player. If anything else is provided, it will return nil.

You can use it like this:

local PlayerService = game:GetService("Players")

script.Parent.Touched:Connect(function(hit)
	--Get the player, returns nil if it doesnt exists, which is why it's in the next if statement.
	local Player = PlayerService:GetPlayerFromCharacter(hit.Parent)
    if Player and Player.leaderstats.Money.Value >= 500 then 
        Player.leaderstats.Money.Value = Player.leaderstats.Money.Value - 500
        Player.leaderstats.Credits.Value = Player.leaderstats.Credits.Value + 1
    else
        print("Not enough money")
	end
end)