There are two ways to reset an Amazon Echo device.
1. You can do a hardware reset. The buttons to press are different on the Echo, Echo Dot v1, Echo Dot v2, Tap, etc. Make sure to read the directions from Amazon. When you do reset an Echo device, it is still registered to the Amazon account that used it previously. But you still have to go through the original setup thru the app. It's similar to iPhones where the owner has to release it from iCloud before it can be used by another person.
2. You can deregister an Echo device from your Amazon account. View your devices here:
https://www.amazon.com/mn/dcw/myx.html/ref=kinw_myk_redirect#/home/devices/1Once it has been deregistered, you still might want to do a hardware reset to get rid of whatever it was that has been causing the problems.