Id like to make Tundra grass in my game brown (it looks red atm). I never edited .dds files. Anyone can give advice how to easily do it?

Have you read this http://www.gamesas.com/topic/1330609-if-youve-released-a-texture-mod-or-plan-to-please-read/ about texture mods in Skyrim?

Try using http://www.gimp.org/, with a DDS plugin like https://code.google.com/p/gimp-dds/.

Paint.NET can also edit dds files. If you just want to make this easy all you'd have to do is open the dds, go to Adjustments->Hue/Saturation, and change the hue to something you like better. Then resave the dds. Shouldn't take more than a few seconds. I'm sure GIMP has a similarly simple option to adjust hue or color.

Yeah, although if Paint.NET can do native load/save of DDS that might be easier. GIMP will load it ok, but you have to remember to export to DDS format.