On the Oracle Data Guard Overview page in Cloud Control, select the standby database that you want to change to the primary role and click Failover. Then, on the Failover Confirmation page, click Yes to invoke the default Complete failover option (from google)
What is failover in Data Guard?
Failover is
a one way process where your primary database goes down due to some reasons and to get back the production live without any loss, you convert your existing Physical Standby database to start behaving as Primary database
please read
Switchover and Failover OperationsNote: The examples shown in this section do not necessarily show the specific attributes you might need to use in your own environment. The required attributes vary depending on your configuration (including whether your environment is Oracle RAC-based or single-instance).
Refer to the appropriate Oracle RAC or Oracle Restart documentation for further information. Database services can be configured to be active in specific database roles on Oracle RAC databases and on single-instance databases managed by Oracle Restart. The broker interacts with Oracle Clusterware or Oracle Restart to ensure that the appropriate database services are active and that the appropriate FAN events are published after a role change. FAN events are always published through ONS.
docs.oracle.com/database/121/DGBKR/sofo.htm#DGBK
