the "Seed Recovery" script is a little program that you can give your "incorrect seed" and an address that should be generated by the correct seed.
It then runs through a few different tests (like swapping words out, commonly misspelled words, or looking for missing words etc), generating a number of "guess" seeds. It then generates addresses for each of those "guess" seeds trying to find a match. If it finds a match... then you've found the correct seed
You would need to download and install "Python 2.7" and then download the script itself (and possibly install a couple of bitcoin related Python libraries that it uses that aren't installed by default with Python)...
If you are not comfortable doing this sort of thing, you would need to find someone you trust and get them to help you out.