Upgrading custom personal builds of ipfire

I've been looking for any kind of documentation on the ability to upgrade / test personal builds of ipfire and couldn't find any.

I build a customized version of ipfire for personal use and obviously can't use the normal network upgrade but i dont see an option to point to a usb drive for a new build or anywhere other than the official servers. What is the preferred way to do this? Is there a way to upgrade offline without clean installing over or some way to set a personal build on a machine as a local repo mirror?

Figure there's got to be a way to test the upgrade process locally..

thanks for any help.