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SelAttributes.Style method of RichEditComponent. You may set bold, italic, underline, strikeout style and other parameters of font in RichEdit. And don't forget to return focus to RichEdit component.

procedure TForm1.Button1Click(Sender: TObject);
begin
  if fsBold in RichEdit1.SelAttributes.Style then
    RichEdit1.SelAttributes.Style:=
      RichEdit1.SelAttributes.Style-[fsBold]
  else
    RichEdit1.SelAttributes.Style:=
      RichEdit1.SelAttributes.Style+[fsBold];
  RichEdit1.SetFocus;
end;