The way you edit decorations depends on your game version:

For Version 1.22.6 and Higher

To edit a decoration, tap and hold it until the green circle stops spinning. You will then be prompted to choose from three or four different designs. Once you make your selection, tap the check mark button in the right corner.

For Versions Below 1.22.6

First, tap the hammer icon in the bottom left of your screen. An icon should then appear over the furniture you wish to change. After tapping on this icon, you'll be able to choose from the available designs. Tap the design you want, then press the check mark button to confirm your selection.