How to replace the original file?
I don't know how openiv replaces the original file
@summerize your question isn't clear. OpenIV can't restore original files unless you have those.
You can restore original files by doing a verify integrity, much faster and generally just as effective as a reinstall. Once restored you can use Windows explorer or OpenIV to copy original files to the mods folder.
If the original files are in an encrypted rpf, then only OpenIV or a similar tool can be used to replace a mod file with the original.
If you mean replacing game original files with mod ones, yes you can do that as well but it is a best practice to use a mods folder.
An important reason for using a mods folder is that if you do need to do a verify integrity at one point, unless all your edited files are backed up, the process will replace/restore all modded/edited files with their original/vanilla file versions.