mejorar
/mexoˈɾaɾ/ · A1 · verb
to make better, improve
mejor = better (Latin melior, as in ameliorate) + -ar: to make better
Mejorar is mejor (better) turned into a verb with -ar. Mejor comes from Latin melior, the same root as English ameliorate, with Latin li becoming Spanish j. Know mejor and you already know mejorar.