To upgrade your UNAC, select workshop, then choose "Expand UNAC", and now you can choose to upgrade your UNAC via money, or uploading operation files.
There are 8 to 14 ranks per weapon part, and each part will take you roughly 8 million just to upgrade. In total, at least 40+ million to fully upgrade. ***Alternatively***, you can use operation files (earned from pvp sorties) to rank up the weapon parts, this option doesn't cost any money. You will probably need to play around 50 online missions to fully upgrade your UNAC. You can also boost this by doing mock battles with a teammate.