That's really easy, I do recommend to download WinRAR on your computer.
Right click on the iso file and select "Add to archive" line from the context menu.
Now in opened window change the archive name, and change the Compression method to "Best" from the combo box.
And at last set the "Split to volumes, size" to how much MB or GB you need. You can do that manually or select the function from the list you need.
You can also set a password to archive files, and some other features available.
Click "OK" and wait until the archiving process finished.
Click "OK" and wait until the archiving process finished.