To take back an Echo, find it in the Echoes section of your dashboard.
Each Echo you've left is listed with the creator, the Canvas, and the reaction you chose. Click it to open the spot on the Canvas, then long-press (or right-click) and choose Remove.
You can also replace an Echo instead of removing it — just leave a new Echo on the same paragraph or timestamp and it overwrites the old one.
If a creator already replied to your Echo, removing the Echo doesn't delete their reply. Their reply stays in your dashboard as a record of the exchange.