Hi,
There is currently no way to rename those tabs. An enhancement request is already filed to display the name of the report document instead of 'Report1', 'Report2', etc. : #12036 (Title of tabs in Report Viewer should list report or 4rp name).
Could that fit to your expectations? Otherwise, what would you like to display instead, or why would you like to rename the tabs?
Regards,
Romain W.