On your old phone you'll want to go to settings>bluetooth>find your fitbit device name>select the blue i next to it>then forget device.
On the new phone you'll want to follow the same steps to verify that your fitbit is listed under your bluetooth devices (which i'm guessing it isn't)
If its not listed you'll want to go into the fitbit app. Since the pairing with the old device has now been deleted and depending on your tracker you should now see a popup asking to connect to your fitbit. This should hopefully fix your issue.
If not, in the upper left corner of the app (your profile picture) you'll want to tap that to get into your account. You'll probably still see your device listed, so tap the device and in the device settings scroll to the bottom and hit remove the device. Next you'll want to select set up a new device.
Hope this helps!
