After snapshot happened, can I sell my coins on exchange or I should keep them on my wallet in order to get armr coins?
You can sell your coins (if you want), you should not keep them for getting armr coins. If your coins were in XCS wallet before 9th Januaryat 1:00 PM GMT+1 you were eligible to claim your ARMR in the snapshot. During Q1 the team will release ARMR wallet, so you'll be able to use your XCS wallet.dat file to claim your ARMR coins.