Convert CBR to CBZ

Both .cbz and .cbr comic books are basically renamed RAR or ZIP archives, so you can also simply decompress .cbr files using any of the available file compression managers (WinRAR, WinZIP, 7-zip etc), repack the images to .zip files and finally rename the extension from zip to cbz so file files will be recognized in any comics books viewer.

However, it should be noted that most (if not all) comic book readers support all of the available comic book archive types (.cbr, .cbz, .cb7, .cba etc.)