Hmm... First, follow the tubing all the way on the input side to see if there's any valve whatsoever. If you don't see any valve, then you're in for a mess.
1) go buy 2 valves for your hoses
2) unplug filter !!!!important!!!!!!
3) place canister in a bucket or container because water is going to flood out.
4) Lift the intake tube out of the tank. The output should be taken out too if its below the water line. Just place the hoses on top of the tank and secure it so it doesn't flop off and spill water.
5) Take the clips off the canister filter that's holding the black upper piece (the one with the hoses attached) with the clear cylinder piece. Then slide the black upper piece off and let the water from the hoses spill out. If you had valves this wouldn't be a problem but... oh well. Be sure that the water in the output hose is completely drained out.
6) Do whatever you need to do with the canister filter.
7) Install the valves that you bought on both hoses, preferably near where the hoses attach to the canister filter. You need to take off any hose clamps that's securing the hose to the canister filter and just pull. Pulling from side to side helps.
8) Put some water in the canister filter if you emptied it, reassemble everything, put the intake tube back in the tank.
9) Now you need to plug the filter back in and quickly suck on the output hose to establish a siphon. Sometimes you need to keep repeating to help out the filter at first.
You should curse whoever installed that canister filter for not installing the "optional" quick disconnect valves.