A “patch” is a compact representation of the differences between files, intended for use with line-oriented text files. It describes how to turn one file into another, and is asymmetric: the patch from file1 to file2 is not the same as the patch for the other direction.
Patches are intended to modify the files — applying the patches adds and/or removes the lines of code to/from the original file. This way, the original file is changed having preserved the main content.
The patch format uses context as well as line numbers to locate differing file regions, so that a patch can often be applied to a somewhat earlier or later version of the first file than the one from which it was derived, as long as the applying program can still locate the context of the change.
Reverting the patch
Step 1. Place the .diff or .patch file to the root directory of Magento or the corresponding module, where the patch was previously applied. The location depends on the instructions received from the Support Team or on the patch content.
Step 2. Connect to the server via CLI.
Step 3. Navigate to the directory, where the patch was uploaded.
Step 3. From this directory, run the command below, where PATCHFILENAME is the name of the patch file you’ve added.
patch -R -p1 < PATCHFILENAME
If Git is used on your server, it makes sense to use the next command instead:
git apply -R PATCHFILENAME
Step 4. Run the commands suggested by the Support Team to make sure that reverted changes are detected by Magento.