For more in-depth steps, these community-vetted resources offer specific tutorials:
"unarc.dll returned an error code 6" typically occurs when the installer is unable to read or decompress archive files during the installation of games like how to fix unarc dll returned an error code 6 fifa 13 link
Delete everything, find a new FIFA 13 repack from a trusted uploader with a working magnet link, and apply Steps 2, 3, and 4 simultaneously. Do not try to "repair" the corrupted file—redownload it. For more in-depth steps