To go back to the beginning, the first thing you do is tap on the star in the upper left-hand corner.
Then...
1. Tap on the little nest icon with the arrow:
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2. Choose the leftmost little screen from the ones at the bottom there—Part 1. Tap the middle of the screen and then tap the checkmark to load the save:
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It works the same way if you want to load any of the other parts—choose the part you want from the ones at the bottom there (parts 1 through 9 in order from left to right).