Quite a simple one, all you need to do is reach level 5 with any of the squad upgrades available in the pre-mission shop. Upgrades are found under the first button in the shop, which has a rifle picture.
You can use command points or medals to complete this, and I would suggest sticking to medals where possible to help preserve progress towards the 3mil command points achievement
For some upgrade types, the first level cannot be purchased with medals, but (in my experience so far) once you've bought the first level with command points you can buy later levels with medals.
Personally I used the increased default rank for the achievement; using only medals I had found enough by mission 3 of operation 2, although you could have unlocked this a lot earlier using command points or by buying medals from the real money store.
One final note - if you use medals to buy the last rank and unlock the achievement, you can force close the game and your medals will be restored when you reload, although the last level will be locked again of course. Useful if you'd like to stretch your upgrade spending power across a wider variety of improvements.
As ever, if you're downvoting please let me know why in the comments so I can improve the solution!