1) you modify the interfered frequency on one of the cell
2) you modify the azimuth (orientation of the antenna) and/or the tilt of the antenna.
Those are the most effective solutions. Aditionnally, you can try:
-Activate downlink/uplink power control
– Activate downlink / uplink DTX (discontinuous transmission)
-Preferably force traffic on a TRX that doesn’t use the interfered frequency (if non hopping)
Finally, you can also hire a RF engineer who will happily implement all that for you 🙂