The upgrade is fairly simple and straight forward, requiring only a reboot. Go to Start and type in 'anytime upgrade'.  All you need to do is enter your Pro license key, wait a few minutes, then reboot. It will NOT affect your warranty or existing data.
I just ordered an XPS 15 Touch, which has Windows 8.1 64-bit English.  I would like to upgrade the machine to Windows 8.1 Professional 64-bit English.  I need to know how to do the upgrade proeprly.  I have a few questions: Is the upgrade a safe thing to do?  Should I do the upgrade myself or get a Dell technician to do the upgrade?  What do you recommend? If I do the upgrade myself and run into issues, who should I contact?  Dell or Microsoft? Does the upgrade affect my extended warranty?
Upgrade to Windows 8.1 Professional