Open Disk Management
It depends from herein, but on any partition that is NTFS, FAT, or exFAT (you don't have to mount system reserved,) right click it, and Change Drive Letter and Paths
Assign it a drive letter using Add
If it's not NTFS, FAT, or exFAT, you're going to have to format it, or find a third-party driver that adds support for the file system type.