To answer your question quickly if drive C is your Boot Partition where Windows resides then no you can not modify it or change it, nor would I recommend you doing so even if you can. Windows and other applications expect certain files to be found in specific locations, if the location has changed then they will not work properly.
As how to change drive letters for the other partitions, right click om the Windows 10 Menu Button, Select Disk Management, right click the partition you want and select “Change Drive Letter”.
Keep in mind that if there is any applications installed on that drive they may not function properly after you change the drive letter. Alternatively I recommend just copying the data on the C Drive to your destination Drive. If you still would like change that drive letter, then yes you will need to reinstall windows, and reconfigure the hard drives accordingly, just make sure your data is backed up elsewhere.
