Open regedit (type regedit in the start menu search box) go to file import and choose the file.
If you removed a number of entries some may not be replaced, and you may want to pre edit it to only replace the entries you want (open the file in notepad or other text editor, highlight and delete any entries you don't want back and re-save the file as a reg file (usually regular save instead of "as save" should keep the file in its original format))
As well it's important to know that, while re-import of reg file fixes most issues, there may be a small, but significant, chance that it may not fix every issue.