New to bicycle riding. Questions about adjustments and upgrades
Leave your current bike alone, save your money for the replacement bike.

To make a significant difference to the current bike you'll probably have to spend a significant fraction of the original purchase price and face major headaches figuring out compatibility of components. Leave it original and sell it when you get the new bike.

If you do want to spend some money on making cycling more pleasant, look at some clothing items or accessories.

A few updates:

One change that you might consider is replacing tires with ones more suited to the surfaces you are riding on. I.e., if you are riding on pavement or flat good quality gravel you can fit semi-slick tires. You don't have to get expensive tires, moderately priced ones will be fine.

Keep the drive train clean, lubricated and adjusted properly to extend the life of the sprockets and chain and keep it as efficient as possible.