First, you'll want to change your site address in the Settings tab in the Weebly editor. Save your changes and copy the IP address you're shown - you'll need this when pointing your domain at 1&1.
There are a couple of things to note with regard to 1&1 domains:
The Weebly IP address should be entered into the IPv4 field, and if there are any IPv6 records set for your domain, you'll need to delete those. If you have any difficulty doing this, please reach out to 1and1 support - they'll be able to get you up and running quickly!