Use Edit->Text->Font Style->Series to change just the font series for selected text areas. To change the current font series to apply to new text areas, use Settings->Styles and select the tab labelled Font.
When you select a font series, FlowframTk guesses at the appropriate LaTeX font series declaration. This is used if you export your image as a LaTeX file. If FlowframTk has guessed incorrectly, you can select a different command from the combo box on the right, or you can edit it if you require a font series declaration that is not listed. Alternatively you can clear the value (by deleting the series declaration) to use the current document font series.