This option is available only as a WUM update and is effective from 6th March 2018 (2018-03-06). For more information on updating WSO2 API Manager, see Updating WSO2 ProductsGetting Started with WUM.
If you want to edit an already uploaded mediation policy,
Select the mediation policy from the drop down list.
Click the download icon next to it, as shown below:
Edit the downloaded mediation XML file and re-upload it.