The way to upgrade an application from PRPC 5.5 to Pega 7.1 is to run the Installation and Upgrade Assistant (IUA) against your PRPC 5.5 Database repository. Once the Repository has been upgraded to Pega 7.1 you can then test your upgraded application to identify any issues that may require fine tuning. If you adhered to Pega Best Practices you should not encounter many if any issues. For more information on using the Installation and Upgrade Assistant in Pega 7.1 please search the PDN for "Installation and Upgrade Assistant" and then download the appropriate Upgrade Guide for the version of PRPC you are upgrading too.
I got your point which you mentioned regarding the application Upgradation from Version 5.x or 6.x to 7.x.. It would be very helpful for me if you could please explain the few more steps from Scratch in order to understand it in a more convienient way which has to be followed while upgrading the Existing application from lower version to a higher version.