How do I remove the yellow hightlight.
Have you looked at the controls in the Sidebar?
You could try selecting the text, clicking Format > Character and in the dialog that opens, select the tab Highlighting and click the button that says None.
If these don’t work for you please complete your question as ebot and the site info request, and also add what you have already tried.
I suggest downloading the Impress Guide from documentation and using that as reference. Cheers, Al