There are 5 Harper upgrades.
Upgrades can only be done in First Aid Tents via the computer inside. The first one you encounter is on the farm.
There are 2 types of upgrades (Crafting and Harper) this achievement is for Upgrading Harper fully
To be able to do an upgrade you will need to find enough "Red Weed Extract" which is generously placed around the environment.
The 5 Harper Upgrades needed for the achievement are:
1: Extra Health (which will cost 30 Red Weed Extract)
2: Extra Carry Weight (which will cost 35 Red Weed Extract)
3: Focus Mobility (which will cost 40 Red Weed Extract)
4: Explosive Radius (which will cost 45 Red Weed Extract)
5: Focus Range (which will cost 50 Red Weed Extract)
You have no choice but to get these in order from 1 to 5. Only the next one in line can be purchased.
It is unlikely that you will get both this achievement and the "Gear Up! - Unlock all throwables" in the same playthrough. I scoured each area thoroughly and had all the throwables, but only 3 of the Harper Upgrades by the end of my 1st game. Luckily there is a New Game+ that lets you start with everything you have at the end of your 1st game. I got enough "Red Week Extract" in my New Game+ before I got to the caves section to complete both achievements.