You’ll first want to perform a GET of the given analysis to retrieve the existing XML definition, then modify the Property fields to include an EvaluationPeriod value, and PUT the modified XML.
Here’s an example XML with the EvaluationPeriod specifying 6 hours:
I’ve done @Aram’s recommendation and I think it’s the easiest way. Also it worth it to investigate the reason of why that content is set to report “every report”.