Sell in order to buy more when the price dips again.
Correction, never fucking sell unless you need the fucking money. I've never sold since I've been been accumulating bitcoin in 2012. I'll never buy a fucking altcoin ever. I count my blessings I never sold, bitcoin has only appreciated.
When bitcoin doubles or even triples in value after the halving, I'll buy this forum off theymos.
I like using price fluctuations to try to increase my trading bitcoin stash (which is small, but still fun to play with). But you're welcome to do as you see fit.