If you mean just changing the image of a tileset, then this is possible in Tiled by opening the tileset (you can do so by pressing the small “Edit Tileset” button below the tileset view) and then choosing Tileset -> Tileset Properties. You can change the “Image” property in the Properties view.
Of course, rather than producing multiple maps or layers for each tileset, you would probably want to do this at runtime in your game (unless you’re using an export process involving a tool like Tiled2Unity, though even then you could probably still adjust the texture dynamically).
If you’re using external image collection tilesets and actually need to exchange one such tileset with another, this is unfortunately not possible yet from within Tiled, but it’s easy to do by just opening the map file in a text editor and changing the
source attribute of the external tileset.