There is a utility called "modify-snapups" that will do what you need: [URL]  Please be extremely careful with it as misuse could cause significant data loss.
A weekly backups was  set to infinite retention instead of 14 days  we have changed it to the correct retention , but we understand this new retention policy will kick in only for new backups?  Question is can we run a report to find out all those weekly backups with infinite retention and then apply a script and delete? what report would I run to get the details of those backups with intention retention
Avamar-Backup delete