Retexturing tutorial?

[Adorno]

I see some fine texturing tutorials for UV mapping/modelling.
But I was thinking it there was one for a simple retexture, specifically ground, houses/roof/stone etc.

So far I have Gimp and dds plugin and figure I need to open the desired texture.
But is there perhaps a step-by-step guide on how to change the existing texture? 

 

[Specialist]

This would really come in handy. I suck at retexturing, I mean, I can barely texture from scratch, and I really need to change a few of the ground and stone textures.....