How to underline existing inner html from jquery

You want to adjust the css(), not the html()

$(this).css('text-decoration', 'underline');

https://api.jquery.com/css/

Off topic again, but @atif123 I noticed you are new here to the forum, Welcome.

Also later on to keep it underlined, say you have to leave and come back later to finish, you probably need another control for a flag, like a checkbox. Then if the checkbox is checked trigger above code to show underlined.

And how are you saving to database. Anyway just post a new question when you get to next step.

typically you would create a CSS class for the underlined text, and then toggle the class rather than amending the style directly..

https://www.w3schools.com/jquery/html_toggleclass.asp

Also there can be quite a learning curve involved here but there are free jQuery tutorials and you can always post code here and ask questions.