The O-ring on your pool sand filter is under the Multiport Valve on the top of the filter. To change the O-ring you will have to remove the Multiport Valve. If your filter was not installed with union connections, you will have to cut the piping to your Multiport Valve. When you reinstall the filter after replacing the sand, this would be a good time to add unions.
To remove the Multiport Valve unscrew unions that connect the piping to the "PUMP" and "RETURN" ports. If your filter does not have unions, cut the piping that connects these two ports. Be sure to leave enough room to attach unions later and stagger the cuts so that one union will not be installed on top of the other.
Roll the new O-ring onto the bottom of the Multiport Valve. Lightly lubricate the O-ring with a silicon lubricant.
With the Valve in "FILTER" position, prime and turn ON the pool pump.