That's a good question as I've never actually done it either
I've only ever filled it back up after draining it all & having the engine removed & refitted so I'm not really sure what's the best way to do it tbh
I'd probably disconnect the pipe at the back of the reservoir & add some hose to it so it can drain into a bottle of some sort, block the hole left in the reservoir and fill it with fresh fluid & run the engine very briefly or crank it over on the key with the king lead removed whilst continuing to fill with fresh fluid letting the pump flush it all through then connect the pipe back onto the reservoir and fill back up & bleed it