I have used LG optimus G pro for like 3 months, its an excellent phone 
There are different types of methods to upgrade LG phones. It all depends on what you want, Either you want a stock upgrade or custom rom with upgrade
1, For stock rom flashing, process is hectic. As you need to follow either KDZ or ToT file method. But once done your phone will be upgraded and fully stock as well. But i believe you wont go over 4.4 with stock image files
Here's a complete guide if you want to go stock: [Guide][Stock Roms] Return Back To Stock AT&… | LG Optimus G Pro | XDA Forums
For customs roms, its fairly easy to upgrade and you have wide variety of roms to choose from. For custom roms you have to follow these steps
Root
- CWM
- Back up your baseband in case something goes wrong
- Download appropriate rom
- Flash it via CWM.
Now if you are looking for Android 5.0 then custom roms are your only option
G pro usb drivers: https://xq55.app.box.com/s/0zdzixtyqcczxd54yx7r
G pro root: https://xq55.app.box.com/s/cg16a92t4ueqg7pbf70h
G pro recovery: https://xq55.app.box.com/s/3y2u1j3e9382l2o8dlkj
