How to modify 3D model of body?
The correct/good method to achieve this is called "rigging", but it is not an easy feat (as pointed out by others), as it requires plenty of knowledge about the software being used to edit the model, and a good understanding of the theory behind it.

Skeletal animation requires the designer to set up a skeleton (also called "rig", hence the slang term "rigging") for the mesh and define the variables controlling the motion range of the joints and and the geometry and deformation of the mesh.

It's quite some job to perform, so - unless this assignment is the first in a series of assignments having to do mesh movement for this model, or you have a keen interest in the topic - I would suggest you to take a shortcut and edit the mesh directly in a "one off" not reusable fashion.

(Rigging would conversely allow you to create a "rag doll" or at least a "rag arm" to freely and intuitively move around as you please).

The tool I would use for either task is blender.  If you want to take the shortcut, maybe a less complex software like meshmixer could also do the job (I don't have direct experience with it, though, so I'm not 100% sure).

The blender foundation have a nice series of videos on the topic, called "humane rigging".