Resizing C: with Bitlocker waiting for activation

Answers with similar keywords all contain parts of my problem, but not all in one, hence I've decided to ask. I have a Dell Inspiron 5390 with Win10 Pro installed in the store. Initial disk partition is 58.9Gb for C: and 400Gb for D:. The OS keeps bugging me with "license will expire soon" and 0x80080005, the cause of which, as I read, is small C: size. When launched Partition Assistant I've found out that partitions are Bitlocker Encrypted, but the Bitlocker itself is waiting for activation. And here is where I got ultimately confused about my next steps - do a backup, activate bitlocker, then disable it, resize C: and activate Bitlocker back? Or formating is a must? Or maybe I should just leave it be and never buy laptops from mainland China ever again? :)