You can modify the standard template for Web Reporting by renaming it and using the parameter _TEMPLATE in URLs:
_TEMPLATE Parameter
If you modify a standard Web Reporting template and rename it, you can specify it in URLs by using the _TEMPLATE parameter.
If you modify the standard selection screen template WEBREPORTING_REPORT and call it ZMYREPORT, you can specify it in a URL as follows:
http://<machine>:<port>/sap/bc/webrfc?_FUNCTION=WWW_GET_REPORT&_REPORT=RFDTST02&_TEMPLATE=ZMYREPORT
When a user clicks on this URL in a Web browser, the function module WWW_GET_REPORT gets and executes the report RFDTST02, and the result is displayed on the modified template ZMYREPORT.
This works for one call, but any links to further function modules produce output based on the standard SAP template.