Remove all HTML labels in a message field

You'll need the Web Mining extension for that. It has the ability to get rid of HTML tags. 

I have to remove HTML labels of an attribute of a dataset.

Which operator should I use?

Depends on how your data is set up but I would look at the Extract Content, Unescape HTML, or Unescape HTML Document operators .

I will try. Can i do it with a regular expressions tha delete everything between <> symbols?

Yes you can do RegEx. Just use the Replace operator. 

What RegEx can I use?

Without seeing your data, I would guess something like this: \<.*\>

and replace with a space or something else.

That's a greedy regex, so that would eat all your tags in one go and leave you with not much content.

Remove tags either with <.*?> (note the question mark that makes it a non greedy regex) or <[^>]+>