Meshmixer is a 3d modeling and design program, but unlike others it is designed for manipulating 3d models which have already been created. It can adjust and re-size models, stretch or skew specific aspects of models, or even incorporate multiple models into a single object for 3d printing.
Meshmizer is free software, anyone can download and install it for free on their personal computer.
Install Link: http://www.meshmixer.com/download.html
Note: Meshmixer was purchased by Autodesk in 2016 and some of its tools were incorporated into Fusion 360. We still recommend the Meshmixer program, as it appears to again be the most easily approachable program for its specific tasks.
There are a number of different user tutorials provided at https://all3dp.com/meshmixer-tutorial/.