Funnily enough, we had the opposite problem with our grey who has always been fine coming down, but wasn't going up  . And it's something that we had to tackle from day one, because our front door is up a flight of stairs.
We overcame the problem slowly, by literally introducing each paw to each step. I'd recommend also holding on to her collar when she starts to walk down as they tend to feel more secure if they know you're there with them. When you try and lead her down, let her go near the wall, or the solid banister if you don't have a wall on one side. Riley seems to want to 'lean' into the wall as he goes down I think this is an attempt to steady himself.

introducing each paw to the step in front will take a while, and you may also need to give her a 'nudge' up the bum - not hard enough to force her to topple down (which will probably make the problem worse) but to encourage the brain to 'move forward' - a tip I got from Cesar   and normally this will do the trick.

Finally, give her lots of fuss and a treat when she makes it down.

Riley learnt the stairs within a matter of days, up and down. Very occassionally he will 'freeze' on them - more often than not when he's got a bit excited and slightly slipped on them so you may have to be prepared to come to her rescue from time to time.