Reimport sprites - how to?

I wish to reimport tons of sprites as I change the overall colour palette of the level. The level is already set up, took 4 days and I have no time to do it again one by one.
When I change the sprites in their location and hit reimport in the editor, it does nothing. I wish to know how to effectively reimport a lot of assets.


what I do is replace the image directly from the folder Assets, outside Unity.
That way you donĀ“t have to slice again, it just replace the image.