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: the Autodesk 123D series of programs is being phased out in early 2017. All of the features of Meshmixer have been implemented into AutoDesk ReMake.
There are a number of different user tutorials provided at https://all3dp.com/meshmixer-tutorial/.