Retrieve the sh3d file your created on your smartphone or tablet, tap Open button (3rd button in the toolbar) in Sweet Home 3D Mobile then Import button and select your sh3d file on your device.
To retrieve it back on your desktop, you'll have to tap Save button (4th button in the toolbar) then Export button. This will save it in a sh3x file you can open under Windows.
See also this page.

Don't hesitate to ask more questions if needed.