What should be my first upgrade? I've been planning to upgrade my Foxter Evans 3.0 (Low budget Bike) to
lessen it's weight. What should I replace to lessen it? Also I'm a 5'11 person
and my frame is a medium. Should I consider replacing the frame due to incompatibility of my height? I only have $400 budget. 

Below are the specs:


24 Speed Gears ! ( 8 x 3 Set Up)
Epixon Fork -- new fork
Shimano Gears
Oversize Bar and Stem
Heavy Duty Alloy Frame Square Tubing
27.5 Bike 5'4 to 5'10
Great Affordable Entry Level 27.5 BikeSpecifications:
Frame: Foxter Alloy 6061 Thick Frame Square Tubing 27.5
Fork: Foxter Suspension with Lock Out
Handlebar: Foxter Oversize Alloy
Stem: Foxter oversize Alloy
Shifter: Shimano 8 Speed Shimano Altus
Brakes: Shimano Hydraulic Brakes
Front Derailleur: Shimano Tourney
Rear Derailleur: Shimano Tourney
Crank: Foxter Steel Triple Chainwheel
Cogs: Taiwan 8 Speed
Tires: CST Jet 27.5 x 1.95
Seat Clamp: Quick Release
Rotors: Shimano Rotor Discs


Thank you for those who will answer.
As everyone above points out, you'll pay double or triple to upgrade piecemeal.  Save up buy a whole bike.  Used if you have to.

The two items that I would consider are NOT on your list:


If you are at the upper limit of your seat height, you might consider getting a longer seat post.  It's hard to pedal if you're not getting good leg extension.  (The frame is probably also too short for your torso+arms, but the legs matter more.)
Clipless pedals and shoes.  Your feet will thank you.  Plus, you can carry them forward to your next bike.


You can also upgrade tires once these wear out.