Not a stupid question! Select some text, pick the Paintbrush icon at top, pick the “Style” tab. In iOS Scrivener the way to get “no style” is to change the selected text to “Use Default Formatting” under the “Formatting Options” link on the Styles tab dialog box.
These older posts may be useful:
Doing a lot of style assignments, esp. Heading Styles, not something I do a lot of as I rely on the structure of the document in the Binder and compile settings to do the heading formatting. Just how I do it.