You can certainly downgrade from 5.26 to 5.20 and I know you can downgrade from 5.20 to 5.18 so there should be no reason why you cannot downgrade direct. If it asks for you email, they give it, or if you havent registered an account with DJI do so, then add your email address and then downgrade. What I have found with changing firmware is that it sometimes takes two attempts. For example, changing from 5.2 to 5.18 it would tell me there was a config error the first attempt and no matter what I did it would not correct itself until I did the downgrade again and then it worked perfectly. So, get the relevant assistant software, click upgrade, lets it do its stuff, cycle the WKM power, and possible try and upgrade again. You should be golden.