How can I replace one skin element with another (for example a button)? Either using an existing one from the library or by a new image.

To replace one skin element with another there are two options:

The first is to replace your image with a new one (or state-based images in the case of buttons). To do this you have to select that element and change its image with another. Simply going to "edit styles" and replace it there:

The second option is to replace the element with another from the library. In this case, you would have to select the element and click on its orange wheel to display the options. Just select the first one, "open the Library to replace this item". This will open the library window and we only have to search for the new element and select it. (From here you could also import a new library element that you do not have included yet and then select it):

If you want to select an image that you have not saved in your library, you should first add it to the skin (inserting it as a button, single image, etc..). To do this you have to select the wheel on it and select the option "Save this item in your Library". Choose family, type and name and click "Save". From now on when you open your Library, you will have that new item ready to use.