Two options.
1. TTE ke paas ja kar unse kahe ki aapko 3AC ki berth chhaiye. Agar berth khali hai toh woh aapko de denge. Aapko jaha se berth milega waha se aapko 3AC aur Sleeper ke difference dena padega.
2. Naya ticket le lijiye agar available hai to. Sleeper ticket cancel kara dijiye.