Hello 263659,
Thanks for using the forums’s, firstly you will want to be sure to have a backup or a clone of your existing storage, I recommend using Macrium Reflect Free to create an image you can restore from if there is any issue.
You can then follow the steps outlined here to remove Windows 7, after formatting the Windows 7 partition you can use disk manager to expand your Windows 10 partition to utilize the entire disk or continue using it as D:\ if you are fine with that.
Let us know how it goes!