To update your nameservers at WordPress.com and have your domain point to Skystra, please follow the below steps.
1. Login to your WordPress.com Domain Management account.
2. Select the domain name you want to update, then click the Name Servers and DNS option.
3. Disable the Use WordPress.com Name Servers option.
4. Input Skystra nameservers:
5. Click the Save Custom Name Servers button to save changes.
It can take up to 24 hours for the DNS changes to propagate (DNS propagation is the process of updating every server across the internet with new information) globally.
You can use whatsmydns to track the progress of the DNS propagation for your domain.