There is a Share button at the lower corner of the screen. Tap on the Share button and you will see a pop-up with an option to Save. You can save your collage by selecting a folder and file name.
Occasionally, you may see two Save options: SD or HD. SD saves in standard definition and the saved image is limited to 1920 x 1080. If you need an image in high definition, you can choose to purchase the HD option which can produce a 2550 x 3000 pixel image. This is suitable for printing on 8.5 x 11" paper.