Rockbox is a free replacement firmware for digital audio players, or MP3 players if you will. It's enhancements include support for pretty much all codecs, advanced sound settings, applications, utilities and games.
Check if your device has a usable Rockbox port: https://www.rockbox.org/.
Install from the official repositories, the official tool to manage Rockbox. Rockbox Utility can install the replacement bootloader, the main firmware, and any extra features like fonts, themes and applications.
Installing Rockbox on your device
The Rockbox project has fantastic install instructions. You can find them at https://www.rockbox.org/manual.shtml.
It is recommended to install the bootloader using Rockbox Utility. For Rockbox Utility to detect your player, it must have write access to the internal disk of the player.
# mount -o uid=1000,gid=1000 /dev/sdb /mnt.
Rockbox Utility can make backups of your device.