The Weaved installer checks our database to see whether the alias you've provided has already been registered.
You can do one of the following:
1) Go to your Weaved account and check the device list. Delete the device using the alias you wish to use from the list. Re-run the Weaved installer. You should be able to assign the alias now.
2) Use a new alias. Eventually, you should go to your Weaved account and check the device list. Delete the device using the old alias from the list. It will be shown as Inactive.