I can't edit the default app for txt documents Android lollipop

[rodion15]

Asus Zenfone 2 with Android 5.0 lollipop.
I find a txt file with File Explorer. It opens with HTML viewer. I go to Settings > apps > and open the entry for HTML viewer, I try to click Clear Defaults, but it's greyed out.
I want to open txt files with an app I downloaded: Fast notepad.
How can I do it?

 

[DelJo63]

Hmm; mobile devices operate differently than a PC.

On a PC, there's an association between the file and the applications (aka the Open With)

On a mobile device, the relationship is reverse; the application controls which files it will access (ie nothing like Open With is available).