Then, click on Change next to the Nameservers:
You'll find two options:
Use Hostinger nameservers (recommended) – use it to point your domain to Hostinger
Change nameservers – use it to specify custom nameservers and point your domain elsewhere
Finally, click on Save.
You can also check the following video tutorial:
If your domain is pointing elsewhere by NS records, your DNS Zone management is moved to the provider you pointed the domain to and should be managed from there