If you install iTunes from the Microsoft Store you have no control over where it is installed. If you download it from apple.com - https://www.apple.com/itunes/download/win64 - then you can potentially change where some of the components are installed, but there is no real reason to do. The main change you should do is store your library on an alternate drive, e.g. by holding down shift as you launch iTunes and then using the option to choose or create a library. A good location is D:\iTunes or D:\Music\iTunes. You can create a library there or move an existing library there before opening it.

See also Relocate iOS device backups for advice on getting your backup data stored on say the D: drive instead of C:. Review the rest of the Make a split library portable user tip for advice on organizing the library on external storage. Note that iTunes still checks the free space on the system (C:) drive before commencing a backup, so if you don't keep enough free space it may refuse to backup, even though the data is redirected to a different path.

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