usavršiti

1. to perfect; to improve
   Za godinu dana je usavršio svoj engleski. — Within a year he perfected his English.
   Inženjeri su usavršili novi motor. — The engineers improved the new engine.
2. (usavršiti se) to improve one's skills; to undertake further training
   Posle fakulteta se usavršila u Nemačkoj. — After university she did further training in Germany.

The imperfective is usavršavati; stručno usavršavanje means “professional development”.