How to rebuild Atlas manually?

I sometimes receive the following error:

Cannot load Atlas During Build/ Enter Game. Please rebuild Sprite Atlas

How do I manually rebuild the atlas? Is that done automatically? In SpritePacker I only have Pack and RePack (which is greyed out)

I recently had and fixed this problem by selecting all my sprites in the project folder and deleting the packing tag in the inspector, then click apply to update changes. This removes all your sprites from the atlases.

Then I re-entered my packing tag and clicked apply to create new atlases.

You can view your atlases by selecting “Window/Sprite Packer” in the Unity toolbar.

This manual page may also provide some insight into how the Sprite Packer works: