How do i open canister for cord retract repair

It's broken. Have it repaired or buy a new one.

Tilt the front of the vacuum to a 45-90 degree angle. Gently tug and release cord until it retracts.
If this doesn't work, repair or replacement of the cordwinder is needed. For this the vacuum needs to be taken to an authorized Aerus Electrolux store.

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Why do you need to get to the cord assembly?
If the cord will not retract, tilt the vacuum to about a 45 degree angle and gently tug and release cord. It will usually retract.
Due to problems ensuing from untrained persons attempting to reassemble the vacuum and having to take it to the shop anyway, I recommend not taking it apart.
If the above isn't the problem, please take it to the nearest Aerus Electrolux store for repair.

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Try tilting the vacuum so the front is at about a 75-90 degree angle to the back. Gently tug and release cord. This will usually retract the cord.

Try standing vacuum on end, then gently tugging and releasing cord. This generally will retract it.
There are many problems that can result in disassembly and reassembly by an untrained person, including shock and fire. For those and other reasons, I don't recommend trying this yourself. Take it to the nearest Aerus Electrolux store for repair.
The back of the vacuum can only be removed by separating the top and bottom portions of the canister. The cord winder will need to be removed and the wires all disconnected. The cordwinder is then taken apart and the cord rewound and set for proper tension for rewinding. If the cord is simply stuck due to having been pulled out too far, this is generally not necessary.

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First, try standing vacuum on it's back end. Gently pull on cord and release. Try this a few times.
If this doesn't work, you'll need to take it to an authorized Aerus Electrolux store and let the service tech repair it. Although it's relatively easy to disassemble, reassembling correctly is trickier. I've had several machines come into the shop disassembled because the owner couldn't figure out how to put it together again.
Also, when taking the cord rewinder apart, there's a risk of breaking tabs that hold it together.

http://www.aerushome.com/storelocator/

try pushing the cord back in whilst depressing the retract button

try a local vacumn repair center or web site

It sounds like the rewinder spring either came off the retainer or broke. If it broke it must be replaced. If it just came off, it can be put back on and rewound. It is a LONG flat piece if springsteel which is VERY sharp. Be Careful.

The trim pieces along the sides just pull off seperating the two housings. The back portion where the retract drum is is held by two plastic tangs on top inside. use a screwdriver to pry the ring down to dislodge the tangs. the bottom part will lift off. remove the front part using two screws just below the hinge. the retract has a long coil spring wound around a free spinning hub. there are two "tangs" inside that engage a ratchet-pawl assembly. you'll need to rotate the drum to rewind the spring onto its hub.