I have installed a cloud app on Azure portal. I have to change DNS name from xyz.cloudapp.net to abc.cloudapp.net in Azure cloudapp.
I'm assuming you're referring to either web/worker role cloud services or Virtual Machines.
You cannot rename
abc.cloudapp.net. You'd need to create a new cloud service
abc.cloudapp.net and then redploy your stuff. If it's web/worker roles, then just push the cloud service .cspkg and related .cscfg to the new cloudapp.net cloud service, and delete the old deployment (and optionally cloud service) afterward. If it's Virtual Machines, just delete the Virtual Machines (but not the vhd's or attached disks), and then create new Virtual Machines in the new cloud service, using the existing vhd's.
If you use a custom domain name, you'll have to map the name to the updated cloudapp.net name.
@Pawan - One of my colleagues described the process of moving VM's to a different cloud service quite nicely at http://blogs.msdn.com/b/sonam_rastogi_blogs/archive/2014/10/31/move-azure-vm-across-cloud-service.aspx Not as easy as you would expect, but works.