I'm afraid we need more information to answer your questions.
1. I just want to test how the new graphics work on the spine skeleton and my question is what is the easiest way to do this? because now it takes a lot of time to change the graphics.
Please describe in more detail what you are doing now, why this takes a lot of time.
Quickly previewing an exchanged texture in a WebGL app created with Unity sounds like a bit of a contradiction. The easiest way would normally be to load the skeleton either in the Spine Editor and exchange the input textures there, or use SkeletonViewer (see here), or exchange the textures in your Unity project and preview the SkeletonDataAsset there.
how can I simply upload the graphics to the given folder and view them in webGL, for example?
Do you mean you want to replace an existing texture of a WebGL project that has been created with Unity? Normal means apply here, there is nothing specific to Spine how you can upload or replace a file on a remote location.
2. how to upload graphics to the game in the simplest and most efficient way in a Unity project?
Please see the questions above, we are not yet sure what exactly you are doing now, what you want to achieve and what your motivation is, why you want to use a WebGL app for that.