How do I make a script that replaces an item in my inventory with another item when I click?

How do I make a script that replaces an item in my inventory/backpack with another item when I click? I made a tool named "Water" and I want to make it so that when I click and it's done playing a certain animation and noise, it replaces "Water" with a tool named "Empty Cup". I'm not good with coding so this will help a lot!