Its about the wheel size I have recently been scammed by a seller into giving me a 20inch wheel rather than a 26inch one. I didnt have time to check and have left that country where i bought it, forever. Now as i am 17 i cannot obviously fit into the 20inch wheels on my mtb bike. My question is can i replace its wheels with 26 inches one? Is it possible? I bought it for a hefty price and i cannot afford to discard it
It is likely the bike frame will not allow you to put 26 inch wheels on a 20 inch bike frame.  They simply will no fit.  Sorry.  At this point, try to sell it as is and get a 26 inch bike.