How to reduce the number of faces for selected parts of a mesh? hello,
I have made a high-poly 3D model by myself. But this 3D model has so many faces and I want to reduce the amount the faces only for some parts of this 3D model. How can I do this in Blender?
Go into Sculpt Mode. 

Pick any brush (for example "draw") and set its strength to 0.

Enable "Dyntopo" from the Sculpt toolshelf and set Density = Brush in the Dyntopo options.

Now, control the size of your brush using F and "Sculpt" on the areas where you want to reduce the number of faces. One quarter of the brush size will be the approximate size of the new faces. Use undo CtrlZ when you need to!