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:
You can change NS records by clicking 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 - and changes will be successfully implemented.
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.
Changes to your domain's nameservers will need 24 hours to fully propagate worldwide. Your website may experience issues during this period.