How do i retexture existing objects?

In my modding project, The Shrine of Rhyanna, I need giant stone blocks. I’ve found that the basalt structure blocks and stones from the Forgotten Gods will work fine, but I’ll have to expand all of their sizes. Fair enough.

But the basalt blocks are dull and gray, but I need mossy and wet.

What is the process for using new textures on existing models? Actually i want a COPY of the models with my new textures.