If you are an On Premise customer you would need to download the report and edit it in BIDS and then re-upload the report.
If you are on Online customer you won't be able to change this. These reports are constructed using T-SQL so if you were to download the report and make changes, you won't be able to re-upload due to the limitation the Online reports be constructed using FetchXML (a bit of a catch 22 in this case).