Upgrade compnents to fix "Everest" brand BSO/MTB
From some quick searching, it seems that "Everest" is a department store brand. Most upgrades that you would really notice would probably cost a significant portion of the value of the bike. Other than making sure it's in good mechanical condition, I wouldn't recommend putting much money into this bike.  Make sure the chain is oiled and there is the correct air pressure in the tires.  Make sure the gears and brakes are adjusted properly and work well.  Even something simple like a set of tires or new brake mechanisms could easily cost $50+.  Just ride it and have fun with it. Save up your money for a better bike.