It's supposed to be easy to change a drive letter to a needed one or assign a new drive letter, except the default C: for system partition/boot partition.
Here, when you can't change drive letter in Disk Management, you'll use this alternative software to continue.
EaseUS Partition Master is a third-party Disk Management equivalent program, which is rated as the top Windows disk management tools in 2020, that overcomes the shortcomings of Disk Management and gives people the other way to solve the most common hard disk problems.