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?
Nicola Sap's suggestion is very good and I would suggest the same.
I guess you have either scanned an object or sculpted it yourself. I am also guessing you are an absolute beginner in Blender.
If you ever find the learning curve in Blender too steep, try Sculptris (free lightweight software) and use the Reducer brush there. Sculptris has got a low learning curve and you can easily learn it in just a few days. I suggest that, only if you have problems with using the sculpt tools and settings in Blender, which daunts most beginners. For some tools in sculpting, I sometimes work back and forth between Blender and Sculptris. I go to Sculptris for mostly its Reducer brush.
First, export the object from Blender. Choose OBJ as the format for exporting. Import file into Sculptris where you can very quickly reduce tris with the Reducer brush, fast and easy. After that, export object to OBJ again and import back into Blender for remeshing or retopology.
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!
You can easily do this using Limited Dissolve either for the entire mesh or selected parts.
Go to edit mode and press X and then click on Limited Dissolve or Click on Mesh --> Clean up --> Limited Dissolve. Then adjust the Max Angle parameter to a desired value so that you don't lose a lot of details on the mesh.
Here is a small demonstration that shows how to go from a sphere with about 123000 vertices to a sphere with only about 12500 vertices if you do it for all of the faces:
After using limited dissolve:
In case you want to do this for only a certain vertices you can go to Edit Mode and then press C and then select the faces that you want to make simpler and follow the instructions above to use Limited Dissolve to simplify them; then press ESC. The result looks as follow: