Definitely leave it fermenting. At that reading it definitely hasn't finished yet!
To keep your mind at rest, perhaps take another reading in a few days time. It should have dropped further by then which will show its still going. Only think about bottling/kegging once you've had the same readings over a few days of testing.
I'm not experienced myself, so not exactly sure what reading you should roughly be aiming for for that particular beer, but 1.020 is definitely still too high! Sounds to me like it's progressing nicely though. My lager started at 1.040 and after a week has got to 1.016. I'm leaving it for another week before I test again.
As I say, I'm not experienced myself, but hope this helps.