How to delete files in another partition

[Albert Wagner]

How can I make changes to another partition with another instance of 
Ubuntu 10.04?  I made changes to a file in /usr/lib/X11/xorg.conf.d on 
installation "A" that caused a fatal error in X during boot.  I 
installed another instance of Ubuntu in partition "B".  I can mount and 
view files on "A" from "B", but I cannot figure out how to modify/delete 
files on "A" from "B".