I am making an assumption here that you already know the normal forward-usage of the patch command. If you’re unversed In case you didn’t want to decipher the entire man page just yet, you’re in luck. I came across an instance where I had to remove a previous patch file from the Magento Enterprise code-base. The patch was originally applied as so:
patch -p0 < firstpatch.patch
Problems ensued, and I was given a new patch, however it was a cumulative patch so it couldn’t be laid over the first patch. So how to remove? After a man-page reading, it’s quite a breeze. You need the original patch to be removed, and use the patch number you used in the first patching instance. In this case it equates to the following:
patch -R -p0 < firstpatch.patch
now you can add the new patch.
patch -p0 < secondpatch.patch
Hopefully this will save someone a man-page reading on a small terminal in a dimly lit server room. 
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