How to replace power steering fluid, Volvo 780/2010 ? plz

My fluid is old, looks dark, Dextron III type was supposed to be red , i read should be replaced every 150k miles .I'm thinking to replace it myself, i purchased filter , 3x 900 ml Dextron 3; i know how to replace filter, but i'm scratching my head with fluid and can't find any tips on internet ./youTube.
It has a hex drain plug on steer box, what about the fluid still in reservoir and hoses leading to tandem pump? should i open clamp on hose at pump and drain it ? i;m scratching my head.
I'm asking because shops are closed here today, i'm worried not to get air in system.
Any tips appreciated!
All the best !