Our vent in our kitchen was So BAD! We took it down, put it on the concrete drive way, & removed the motor. My husband cleaned the outside of the motor housing. We then sprayed the vent hood, motor blades & all the fasteners with a degreaser. Let it sit until it looked like the build up was soft. Rinsed it off, used Daun & the degreaser in water & scrubbed. Everything came off. We rinsed & dried in the sun. Then up it back up. Worked!!
The above is what we did too. But I took it one step further. I got some Rustoleum spray paint and sprayed the outside for a newer look.
You may have a screw fixing that will release the filters so they can be replaced.
Hope that's of some help.