The level system was a feature that was given to everyone who started the game between 2 certain dates, and it was also given to some people after a certain update. There is no way to get it if you don't have it and the only way to remove it if you do have it is to do a hard reset of the game. (Disconnect your Game Center or Google Play account, then delete and reinstall the game.)
Details: Players who have the level system will either need to earn or spend a certain amount of cash to get the upgrades instead of buying them with cash. You will get them for free after a certain amount of cash is earned or spent. (For some players, it's earn, and for others, it's spend.) The amount of money earned or spent does not increase if a new level can be reached until the level is clicked on. The level does not reset after claiming angels or even after beating Earth and restarting it in the stats menu. Players who have the level system have higher requirements for upgrades than those who don't have it. In the beginning, the differences are small, but become ~exponentially bigger as you progress through the game, exceeding over 10,000x the normal upgrade prices near the end of the game.