How remove curly braces {} from all string of a json

You could use a regular expression replace:

var jsonString = '{"caseFolderId":"{2854EE70-146A-4618-826C-BA33FC807A25}","caseTaskId":"{28BE9F56-A7D7-4CA5-B777-A51FF3847236}","stepName":"Item details","columns":{"DV_NAMECL_POS_ODA":"Tieleman Transport BV","DV_MATNR_TXZ01_POS_ODA":"445811 - EUROPRENE SBR 1500 GUC BOX 1110 P150 - 1","F_StepName":"Item details","DV_Caseidentifier_for_eni_OdA":null,"DV_MENGE_MEINS_POS_ODA":"20.000 - KG","DV_PEINH_POS":"1000 - KG","DV_NETPR_WAERS_POS_ODA":"1.000 - EUR","DV_EINDT_ODA":"2015/11/28","DV_EBELP_ODA":"00010"},"DV_Caseidentifier_for_eni_OdA":null,"DV_EBELP_ODA":"00010","DV_EINDT_ODA":"2015/11/28","DV_MATNR_TXZ01_POS_ODA":"445811 - EUROPRENE SBR 1500 GUC BOX 1110 P150 - 1","DV_NAMECL_POS_ODA":"Tieleman Transport BV","DV_NETPR_WAERS_POS_ODA":"1.000 - EUR","DV_PEINH_POS":"1000 - KG","DV_MENGE_MEINS_POS_ODA":"20.000 - KG"}';
jsonString = jsonString.replace(/("{|}")/gi,'"');
console.log(jsonString);