The upgrade quests are unlocked after
and consists of varying tasks. There are a total of nine quests, and they must be done in solo.
These missions are very easy and consist of various tasks including defense, finding weapons, and hijacking trucks, among others. Bring good weapons and some health syringes and you'll be good to go.
The most difficult one for me (took a few tries) was the quest where you have to defend yaks from waves of snow leopards. The yaks will attack you if you agitate them in the slightest. I used the D2 (double barrel shotgun) and only got close to kill the leopards.