About the only thing you can really do is attach a hose to the drain cock at bottom, run it outside, and turn it on full with the supply on. If it’s an electric heater, sometimes you can take out the bottom element and try to dislodge some of it from there and flush it out afterwards. If there’s a better way then hopefully someone else will come along and add it here, but I usually don’t mess with them too much if a good flushing does’nt solve it….if the sediment is too built up or is too hard to be flushed, it’s just time for a new heater.
I turn off water and el. drain tank down and take out bottom El. Take a wet vac and tape a piece of water hose on to vac hose and put in el. hole and vac it out, have to break the stuff up too. Put every thing together and fill up and turn on hose bib and flush what ever comes out.