updating music to DRM-free version

Are these original iTunes Store files from years ago?

Up until 2012 you could pay a supplemental charge and upgrade to non-protected, 256k AAC "iTunes Plus" but that is no longer available. Depending upon how many files are involved you can investigate the cost effectiveness of several options.

1) Burn them to a CD which will then perform as any regular CD would for playing and, uh, other purposes. ;-) If you do decide to rip and encode sound files into a lossless format such as AAC or MP3 there will be slight quality loss upon every encode to a lossless format.

2) Subscribe to Apple Music ( Join Apple Music on your iPhone, iPad, iPod touch, Mac, or PC - Apple Support ) or Match ( Subscribe to iTunes Match - Apple Support ). If they are still available this this will let you re-download them in an unprotected format. Delete the protected format ones first (after backing up to be safe) then re-download ( Redownload music - Apple Support ).

If you're referring to modern Apple Music tracks, this information from an older post may still be valid:

Apple Music: How to Purchase A Song After Streaming in Apple Music - Apple Music: How to Purchase A Song Afte… - Apple Community - "Highlight the song, click on the three dots ... click on "Go To" > click on "Show In iTunes Store" > click on the buy button. You can buy the entire Album or just the Song. "... "Apparently, I had purchased the song from an album and also had an "Apple music" copy. Once I removed the "Apple music" copy from the playlist, I added the purchased version to the playlist and it worked."