Red Hat does not support in place upgrade. More information is available in this SAP note 1496410 - Red Hat Enterprise Linux 6.x: Installation and Upgrade.
To update the system to EHP7 you need to have a supported OS and DB. Check PAM for the supported OS and DB. In your case you will need to setup the target server with RHEL 6 and perform a Homogeneous system copy. If required you will need to upgrade the DB to the supported target level. You will need to install SAP and DB software on the target system and then take a full backup of the database on the source system and restore it on the target system running on RHEL 6. As you are on Linux you can avoid the SAP and DB installation by doing a file system copy (SAP and Oracle binaries) between the source and the target server using rsync feature. I have done it for SAP with Oracle on UNIX/Linux systems and it worked. This is just an alternative for the SAP and DB installation. Once you have the source system setup on the target server you need to create a stack file and start the upgrade to EHP7.
RB