smenjivati

1. (from a post) to dismiss; to remove from office
   Nova vlast stalno smenjuje direktore javnih preduzeća. — The new government keeps dismissing the heads of public companies.
2. (on a shift, duty) to relieve; to take over from
   Svake noći u ponoć ga smenjuje kolega. — Every night at midnight a colleague takes over from him.
3. (smenjivati se) to alternate; to take turns
   Danas se sunce i kiša smenjuju ceo dan. — Sun and rain have been alternating all day today.
   Roditelji se smenjuju pored njegovog kreveta u bolnici. — His parents take turns sitting by his hospital bed.

The perfective is smeniti.