The HEPA filter in your Hoover vacuum is a lifetime filter; unless it is damaged, it will never require replacement. It does, however, require periodic maintenance. Although the HEPA filter cannot technically be washed, it can be cleaned out.
Turn the power to your Hoover off, and unplug the power cord from the wall.
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Remove your dirt canister by pressing the small button on top of the canister. Once you press the button, slide the canister out of the vacuum.
Remove the cover from where the bottom of the dirt canister was. You'll need to unhook the latch to remove it.
Remove the HEPA filter from its mount. Bring it to a trash bin, and carefully tap it against the inside wall. Rotate it a few times while you are tapping it. You'll need to do this a few minutes to properly clean the filter.