under "about this device" it says, OS: Ubuntu 15.04 (OTA-15)

EDIT: I have the UBports installer running and it is asking me to reboot to bootloader. I am following the instructings, and the phone says it is entering fastboot and the initial screen, then it doesn't and switches to normal boot on its own. Is there any way to reboot in bootloader mode from the terminal?

Hello, the phone above as a secondary phone. It runs 15.04. The (old) app store no longer works on it and i haven't been unsuccessfully getting open store installed on the phone. I would like to upgrade the OS to what ever the latest the UBports community using. Sorry if this is a basic question, but can anyone help me out?

Thanks!