Need Help, Possibly Simple Hair Edit.

Hello there, first thing to change hair is not so simple, short hair like you want is more easy but still.
first thing you got this 2 tutorial on this forum:
www.sims4studio.com/thread/917/basic-hair-texturing-photoshop-cs6
www.sims4studio.com/thread/134/mini-tutorial-sims-hair-components

start with reading this 2.
now if u want to really change the hair you will need to learn the following:
1>how to use blender
2>how to edit or create mesh
3>UV editing
4>sims 4 bone assignment

this are the basics, you will also need to know how to use sims 4 studio.
about hair look like a paint, first for it to be 3d u need to change the mesh, BUT IN YOUR CASE OF THIS SHORT HAIR YOU DONT HAVE TO.
to make a very short hair you dont have to mesh , but you need to also edit the rles files, for shadows, reflections and most important for you the bump mapping, bump mapping give you the feeling of a 3d even you use only 2d image.
you got tutorials to learn all the above in this forum tutorial section.

NOTE: i know it sounds alot, and it can be scary, but to be scared, its not so hard to learn most of it, and you can start will only learn how to modify the bump mapping file and it will be good start for you.
soon i will publish a tutorial that will give lot of details on uv/bump mapping/shadows and reflections, if you need more help you can always reply here, or send me pm, i be happy to help.