I Googled this, and found the following in a Reddit post:
"On the Alexa App click on your dot. If you grouped it you may have to click that then on your dot.
Select 'Bluetooth Devices '.
Select 'Pair A New Device ' it will start searching for a new device.
On your firestick scroll over to 'Settings'.
Select 'Controllers and Bluetooth Devices'.
Select ' Other Bluetooth Devices'.
You should see your echo dot. Select that."
That should do it!
Stephen