You can't. While there are no dead ends in the game, there is also no restarting on the same account. Don't worry - even if you make some mistakes in the beginning, you can always develop your Base up to a higher Level and complete all of Mr. Black's Tasks. Be sure to follow the instructions in the order that they're given or you'll slow down your Base's development by using up your Resources too quickly. If this happens, just wait until you collect enough to continue.