How do I report chapter on mobile device?

Same way as in the web version

Click in the manga name to go the manga main page and look for a little box with a white flag on the left side of the screen. You can try to scroll up and down to make it easier to spot it, if you have a bad eyesight.

Then you write that someone replaced a good chapter with a really bad one and that the person was probably trolling.

I'm not seeing the little white flag ppl are talking about

I'm on mobile and it shows for me.
You need to be on the Manga page where it shows the name, rating, chapters and look for It on the left side of the screen. It's a bit tiny but you will find it eventually.

Found it. Thanks!

You're welcome ♡