Step by step:
1. Turn off pump
2. Switch Multiport Valve (MV) to "Backwash"
3. Turn on pump
4. Backwash until water coming out is clear (should have a sightglass otherwise watch waste hose for water color).
5. Turn off Pump
6. Switch MV to "Rinse"
7. Turn on pump
8. Rinse for 10-20 seconds
9. Turn off pump
10. Switch MV to "Filter"
11. Turn on pump

Pressure should return to normal, return valve should have good strong flow coming out of it.