The Alexa holds a copy of the devices in local storage. This speeds up interactions with your Smart Home. The downside to this is that the Alexa can't see when you update devices in the Smart Voice Portal.
If you have only added devices and not modified then you say "Alexa, discover devices" to learn in new devices.
However if you have update the configuration of a device or changed a name and the old name still exists, you need to manually delete it. The Alexa Web portal has the ability to Forget All devices.
Smart Home -> Devices -> Forget All
The Apps on both iOS and Android only have the ability to Forget Devices on a one by one basis. To achieve this follow the following
- Open Alexa App
- Open the Menu by pressing the 3 lines (hamburger)
- Select Smart Home
- Tap on the device to be deleted
- Tap the Verticle Elipse (...) in the top right hand corner
- Press the Bin icon in the top right hand corner
- Confirm by pressing delete from the dialog that pops up.
Alternatively you can unlink and relink the skill which will delete everything.