Welcome back! Connect the phone to your computer via USB, then swipe down the notification panel on the phone. Tap the USB icon and make sure MTP (or File Transfer) is selected.
Then open Windows Explorer on your computer. The phone should be listed in the device list. You can now explore the phone's storage. You can also drag and drop files from the computer's hard drive to the phone's storage. I recommend copying the music files to the preexisting Music directory. Also, be aware that not all music formats may be compatible. MP3 and M4A/AAC (as long as they do not have DRM copy protection) should work fine. WMA isn't always compatible.