Ugrades via the installation/rescue ramdisk kernel (bsd.rd) are simple when you have access to the console. On remote machines, however, such access can be complicated, or impossible.
The upgrade guide describes remote installation procedures. Upgrade guides are unique to each release, and for 4.8, the guide can be found here:
http://www.openbsd.org/faq/upgrade48.html
Follow the instructions for
Upgrading without install kernel.