Attach a script to Slot1

Call it inventory_slot.gd

Godot has built-in functions for dragging and dropping

Create the get drag data function

If the slot is empty exit the function

Setup a preview texture
Create a new  Control node
Add a texture rect as child of this node

Move the texture to the centre of the mouse cursor
Pass the icon we made to the built-in set_drag_preview function

Return the data so that we can swap items later

Create a function to check

If data being dropped is valid

It has to be an origin node

Finally, drop the data

By swapping the textures

Populate Slots