Change NS records of existing domain in infinity

How can i change the NS records of the existing domain in infinityfree account, i have upgraded to premiuim account in ifastnet, i am unable to get the option to change the NS records in infinityfree account, can anyone help me in this?

You can ask iFastNet to migrate your domain from InfinityFree to them.
Also, Nameservers should be changed on your domain registrar, not on hosting.