In Menu->Open/Share Solutions, you should see in Local Solutions all saved solutions from the current puzzle and from other share-enabled puzzles.

If you didn't save any solutions from the previous puzzle, you can easily go back and save your final solution from that puzzle. In the Puzzle Menu, you can find the old puzzle by typing the puzzle number in the search bar. Remember to check the box beside "Show expired puzzles" beneath the search bar.

In the old puzzle, you can do any work you like and save solutions as you normally would, using Menu->Save Solution. When you return to the current puzzle, those solutions should be visible in Menu->Open/Share Solutions.