The guide still links to the 17.12.1 switcher file, but says you should then unzip the fp2-sibon-18.01.1-manual-switcher.zip, so download that in the first place.
Alternatively, there should be some link in the Updater App (don’t know that, never used it, sorry) for advanced or experienced users which would let you switch to Fairphone Open OS without doing too much manually. Or perhaps try to open the switcher file with the Updater App directly.
@Lidwien has a point, I use TWRP for updates myself, even if it means more work.
But if you want to switch OSes that way, do yourself a favour and do a really clean install of the OS … else if you run into any trouble later, you’ll think “Ok, I could have made a clean install from the start to rule out problems with the switching process, why didn’t I do this?” .