Dirt Devil upright canister with HEPA filter is loosing suction power?

I can't find my vacuum paperwork to troubleshoot myself. I found a clog in the lower hose coming from the floor brush and cleared it. I have never changed the filter, maybe it is clogged ? I have had the the vacuum over 1 yr and am not sure which filter i need to replace. It is a Dirt Devil Upright Canister vacuum with the HEPA filter, Ultra Vision Turbo (Green in color). I'm not sure which filter i need to get to replace the old one and maybe that is not the problem ? I am a single mom and don't have anyone to help me with it.