Every time you complete a task that has a decoration associated with it, you’ll get a choice of three styles of decoration to choose from. Once you make your choice, you can change this decoration at any time by tapping and holding (long-tapping) on the decoration you want to change the appearance of. 

The first decoration you choose upon completing a task is always free, changing the decoration’s appearance after the tasks completion costs coins. After you buy the style, however, it’s yours to keep forever!  

Have fun mixing and matching various decoration styles on your island!