Short Answer:
Conjugate the verb preferir in all forms in the Present Tense.
What takes place in the conjugation that makes this verb a stem-changing verb
Yo prefiero
Tú prefieres
Él/Ella/Usted prefiere
Nosotros/Nosotras preferimos
Vosotros/Vosotras preferís
Ellos/Ellas/Ustedes prefieren
The stem-changing occurs in the third person singular and plural forms (él/ella/usted, ellos/ellas/ustedes) where the e in "prefiere" changes to an i. This is known as a stem-changing verb because the vowel in the stem of the verb changes when conjugated in certain forms.