You can't. There are no dead ends in the game, but 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 Sector up to a higher level and fulfill all of the General's Tasks. Be sure to follow the instructions in the order that they're given or you'll slow down your Sector's development by using up your resources too fast. If this happens, just wait awhile until you collect enough to continue.