MeshLab (http://www.meshlab.net/) is an amazing open-source tool for processing and editing 3D triangular meshes. What we need here is its ability to cleanup and reduce triangle numbers.
Download and install the software
Open the software
Import (File > Import Mesh) the 3D mesh you need to simplify
The imported mesh in MeshLab
You are now ready to simplify the model:
Select, from the main menu Filters > Remeshing, Simplification and Reconstruction > Simplification > Quadric Edge Collapse Decimation
You will get the following simplification window:
The Quadric Edge Collapse Decimation window
For our purpose, we just need to decrease the number of faces
Lower the Target number of faces value (it is usually already halved) or use the Percentage reducion value (0 to 1)
Click Apply
Repeat the process until you are happy with the results