Pretty simple achievement. From the main menu, go into Inventory and hit the cn_RB to go over to "Class Skills". There you will find all of your skill cards that you have. The skill cards that can be upgraded will have a blue arrow and "Ready to Upgrade" on them. Press down on the cn_RS and it will upgrade the card. You will need a certain amount of duplicate cards to upgrade each card and you can get cards through the loot boxes that you earn or buy with your credits.