Is this the one you have: http://home.att.net/~ctbarbarin/files/57af_1.jpg
If it is, disconnect the hose from the canister (there is a locking lever on the bottom side of the hose near the handle of the canister), lift up on the canister's lid/cover latch under the handle of the canister, slide the cardboard ring on the filter out of its retainer.
HTH,
Conan Kelly, November 2007
