Usually the vent cover pulls straight down, and is held in place with 2 V-shaped metal spring wires.
Squeeze each spring wire together to release cover from ceiling box.
Use vacuum to clean vent fan housing. Some vent fans can be disassembled and motor housing removed for cleaning. Do NOT use water on electric motor. Simple vacuum, or dry toothbrush cleaning is good enough.
Spay tiny bit of WD40 on the fan motor shaft or add tiny dab of white grease if desired.
The plastic cover can be scrubbed with soap and water, rinse to remove soap residue, and when dry, paint with aerosol spray paint using 3-5 very light coats applied in 5 minute intervals.
If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7