As above, pour out the water and dump the rest in them garbage. Rinse out the reactor.
When you go to rinse out the GFO media, you can buy what is called a plankton collector. Florida Aqua Farms sells them, but any place that sells stuff for culturing live food will have them. It looks like a big cup with the bottom replaced with a very fine mesh. I use that to rinse the GFO media. Just dump it into the cup and rinse it. Only the dust gets removed, after you can put the media into the PO4 reactor.
If you want to buy the collector from FAF here is the part number.
BB-PC 4" dia cup, 53?
Also, I like to add carbon to the reactor. I add it on top of the GFO with a sponge in between.
HTH