You can order a replacement HEPA filter online from Amazon and there are third-party units available but my experience is that they don't all fit the models they're supposed to fit.

The Dyson replacement HEPA filter is under $10. Go for that one. Dyson says their HEPA filter lasts forever. Well it doesn't because it simply gets dirty and clogged

Once you remove the canister, immediately below the canister locate the HEPA filter cover. There are three clips. Gently use a screwdriver to lift them up.

Remember there's one in front and one on each side

Left out the old filter [not pictured here] and place in the new filter

Using your finger replace the top cover until the three clips snap in place. That's it you're done