New to bicycle riding. Questions about adjustments and upgrades I just got my very first bicycle. At 30years old, I never ever rode a bicycle. I walked into a Walmart, bought the nicest looking mountain bike I could find, a 26" Havoc Men's Mountain Bike.

My first week was horrible. Trying to balance on a bicycle, falling off several times etc. Second week, I would almost die of exhaustion from riding 400meters. 
Now, after about 6 weeks, I do about 5miles a day with ease. 

I have now realized from a lot of the comments here and on YouTube that I was really using an entry level piece of metal. I have plans for a huge upgrade but while still learning to build endurance, understanding the little bits and pieces of increasing speed etc, any advice on what I can do right now to upgrade this present bicycle to make it more efficient? Anything I should consider doing?Anything I may have left out?  Any helpful tips etc. 

Thanks.
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.