There's lots of ways down from Euston to Crystal Palace and back. Crystal Palace runs various train routes and is on the Overground.
Easiest route is probably get the Northern Line from Euston to London Bridge, which is 6 stops/12 minutes. Then a train direct from London Bridge to Crystal Palace every 30 minutes. That train takes 22 minutes.
There's a few other ways too, so if you missed the train you'd have options.