The U12 Plus has a microSD memory card reader. If your personal computer can also read that card type (Most notebooks are so-equipped), you should be able to save music files (The HTC U12+ supports these formats: Hi-Res Audio certified) from your computer to a microSD memory card, and then, from the memory card to your HTC U12+. Your owner's manual should tell you how to assign the musical file as your phone's main ringtone.
The HTC U12+ is an Android phone and there are plenty of free ringtone apps such as MP3 Cutter, Zedge and more.
More on free ringtones
Here are links to some of the 100% free ringtones (HTC U12+-compatible) offered here at MartinsRingtones.com:
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