what to do if network connection breaks between an upgrade

[Tapas Mishra]

I edited the sources.lst changed lucid to maverick
 and did apt dist-upgrade
it connected and started doing every thing .
I have plans of doing this on other systems also in case if some thing
goes wrong such as a power cut or
network connection errupts in between or say I had to shutdown the
machine or machine did reboot for some God knows reason
then what step should I take to resume the upgrade from where it
stopped or can I undo this entire thing.