You can manage your DNS records via DNS Zone Editor. To reach it, go to the Domains section and click on the domain for which you want to change nameservers:
To change the NS records, click on Change on the overview page:
If you want your domain to point to Hostinger via Nameservers, just choose "Use Hostinger nameservers (recommended)":
If you want to use external nameservers, click on "Use my own nameservers" and insert the values:
Click Save to apply the changes, wait up to 24 hours for it to be fully propagated worldwide and after that, your domain will be pointing to your new nameservers 😊
NOTE:
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
Additional resources: