If its my turn and opponent play rengar to starspring can i play a unit there to trigger starspring.

Yes — Star Spring's "first time a player plays a non-token unit here each turn" is tracked per player, not once for the whole table, so your opponent's Rengar play doesn't use up your trigger. As long as you haven't already played a non-token unit there this turn, your unit's play triggers it for you and you choose whether to move one of your other units there back to Base (FAQ #12439; the "they may" makes you the trigger's controller).

Two things to check before you count on it: the unit you play must legally be able to enter Star Spring — you must control it, or have a permission like Ambush to play there, since being able to respond doesn't let you play to a battlefield you don't control (Rule 355.2.a). And the timing has to permit the play: if the Rengar play opened a Combat/Showdown, you'd need Action/Reaction permission rather than a plain Main Phase play (Rule 806/813).

Also note a move into Star Spring is not a "play," so only an actual unit play consumes the first-time trigger.