How can i use BLP files?

i import them, then how do i change the MODEL FILE for a unit?

.blp is a texture, for it to work, you need to import it to the same directory as the model file it's based off, thus overwriting the old texture.

You cannot open .blp files with paint tools such as paint shop pro. But you can open them with other Warcraft III tools such as Image extractor. Models are never .blp files, .blp files are like images but different type.