By default, apps downloaded from the Windows Store are automatically installed at the root of your system drive (in C:\Program Files\WindowsApps). However, if you are running out of space on your primary drive, Windows 10 gives you the ability to move your apps to another drive.
Go to Start > Settings > System > App & features and launch a search by typing the name of the desired app into the Type an app name field. Once found, select the app from the list and click on the Move button:
Select the destination drive and click Move:
N.B. If the Move button is grayed-out, it simply means the app can't be moved around.