Which version of 9.1 are you running? ("uname -a" command should show you that easily)
A) If you are running the "original" 9.1 from the end of 2012 (no pkgng support), then a full re-install is the recommended method of updating.
B) If you are running one of the 9.1 rolling-releases (old development track with initial pkgng support), then you should be able to point your system to the CDN and update it using our pkgNG repository before proceeding to C.
NOTE: a full re-install might be simpler in this case as well - due to the large number of changes to the backend systems and boot environment support.
C) If you are running 9.1 Isotope Infusion (the latest 9.1 that was released), then you can manually run the "pc-updatemanager" command to list and change to other PC-BSD branches (such as 9.1-RELEASE to 9.2-RELEASE). "pc-updatemanager branches" will show what branches are available, and "pc-updatemanager chbranch 9.2-RELEASE" will change you over.
~ Ken Moore ~