Preferir would be a e to ie stem changing verb, which means you would have to replace the e in the middle with ie, making it conjugate like this:
Yo - Prefiero
Tu - Prefieres
El/Ella/Usted - Prefiere
* Nosotros - Preferimos
* Vosotros - Preferís
Ellos/Ellas/Ustedes - Prefieren
Note that your wouldn't replace the e to ie for Nosotros and Vosotros for any stem changing verbs, just conjugate them as you normally would.