Thank you for reaching out to the Samsung community! To set MX player as default video player navigate to settings>>Apps>>3 dot>>Default apps>>Set as default>>Select MX player. You can also do reset app preference from Settings >> Apps >> Tap on three dots >> Reset app preferences. Go to gallery tap on video file and select MX player.
Be sure to hit ♡ when you find an answer that works for you. For Further assistance, you can get in touch with us via Live Chat option by following- http://livechat.support.samsung.com/Customer_new/IN. You can also connect us via Samsung Members web community by following mentioned url-https://community.samsungmembers.com/en_IN#/
