Thank you for the links!
This does it:
set to bold
RichMemo_Notizen.SetRangeParams(RichMemo_Notizen.SelStart,RichMemo_Notizen.SelLength,[tmm_Styles],'',0,0,[fsbold], [])
set not not bold
RichMemo_Notizen.SetRangeParams(RichMemo_Notizen.SelStart,RichMemo_Notizen.SelLength,[tmm_Styles],'',0,0,[], [fsbold]);