1. # the following command produces (given two source trees ~/original/sources and ~/updated/sources) an overview of the differences between these two trees:

    cd ~/original/sources
    tar -xzvf ~/whatever.tgz
    cd ~
    diff -ruN original/sources updated/sources > how.to.patch.original.sources
   

2. # the following command produces, given the original source tree and the differences between the original and updated sources, updated sources:

    cd ~/current/sources
    tar -xzvf ~/whatever.tgz
    patch -p2 < how.to.patch.original.sources
   

The people sensible enough to give good advice are usually sensible
enough to give none.