You can sell decorations you no longer want. To do so, simply activate the relocation mode by tapping on the button with four directional in your menu bar at the bottom of the screen:
Then select the decoration you'd like to sell by tapping on it. After that, tap on the violet bulldozer button at the bottom of the screen and confirm in order to sell the corresponding item.
Keep in mind that some items cannot be demolished - these include useful items such as the Marketplace or some limited time event items.