When you begin to have multiple subscriptions based on Pay-As-You-Go offer, it becomes a nightmare to manage it in Azure portal because by default they all have the same name. In order to change the name of your subscriptions and reflect why there are used for, connect to your Azure portal and go to subscriptions menu. Select the subscription you want to modify and click on Manage.
It will automatically redirect you to your Windows Azure subscriptions management page (you can also access it from here). Then you can edit subscription details to rename it as below.
Once it’s done, you will need to wait a few minutes to see the change in your Azure portal.