How do you convert lossy music to another lossy format? I'm having my music encoded in 96kbps opus in my phone and it is much more convenient to encode them fro

How do you convert lossy music to another lossy format? I'm having my music encoded in 96kbps opus in my phone and it is much more convenient to encode them from flac than another lossy format. I have hundreds of albums bought and ripped to flac on my NAS server. They take about 300 GB out of the 40something terabytes available. I have real rarities stored in there and I pay about 1 euro a month for the cloud backup.