On the back of the unit to the side, there should be a recessed button (not to be confused with the button on the bottom that holds the battery cover in place). Press that button and a menu should appear. The first option is LOAD and the second option is RESET. Select RESET. It'll ask you to confirm your choice. Top is YES, bottom is NO.
Just to be clear, resetting will wipe out everything and you will start completely fresh so any accumulation of generations, characters, items, accessories, Gotchi Points, etc. will be gone. Loading your Tamagotchi will resume your Tamagotchi's progress from the last point of evolution/connection without starting completely fresh.