Basically you can't. That's becasue, and assuming of course, that this an OEM OS that was preinstalled by the manufacturer. So you can only update 8 pro to 8.1 pro through the store. If later on down the road the PC gets dorked up and reinstall from the recovery partition will take it back to 8 pro and then you will need to upgrade to 8.1 through the store.
Otherwise you'll need a new product key for 8.1 for a clean install of windows 8.1.
As a work around you can use backup software* to make an exact image of the hdd which includes all the installed programs and windows keys onto a new hdd of the same or larger size. Standard practice is to put the old (original) disc in a cupboard or safe and use the new one … in the event of hardware or virus disaster you can replace the old hdd and boot back up in the time it takes you to screw in the new drive.
* I use Acronis true image but there are some free softwares around that are used by the folks on this forum and for personal usage they manage to do the job.