poboljšavati

1. to improve; to make better
   Grad stalno poboljšava javni prevoz. — The city keeps improving public transport.
   Svakog dana poboljšavam svoj srpski. — I'm improving my Serbian every day.
2. (poboljšavati se) to improve; to get better
   Vreme se polako poboljšava. — The weather is slowly getting better.
   Njegovo zdravlje se poboljšava iz dana u dan. — His health is improving day by day.

The perfective is poboljšati.