Being SSRS reports are in Application suite we can't modify directly in your model.
For this below steps may helpful to modify existing SSRS reports.
1. Create new model with selecting existing package as "Application suite". (Overlayering)
2. Right click on your report -> Customize.
3. Open report. Right click on design -> Edit with VS designer.
4. Do your changes/ modifications.
5. Right click on report from your project.->Deploy report.