If you need to manually remove specific backups, you can do so using the PowerShell cmdlets: Get-RMADBackup (with some filtering) and Remove-RMADBackup.
For example, to delete backups older than 7 days (regardless of collection), you can type something like this: $date = (Get-Date).AddDays(-7)
Get-RMADBackup -MaxDate $date | Remove-RMADBackup -Delete $true -Confirm:$false
The retention policies are bound to a collection. So, to run a script on backups from a collection, you need to specify the collection ID: $date = (Get-Date).AddDays(-7)
Get-RMADBackup -CollectionId [id number here] -MaxDate $date | Remove-RMADBackup -Delete $true -Confirm:$false
To get the collection ID, you can run the Get-RMADCollection cmdlet: Get-RMADCollection | select Id, Name, ConsoleSideRetentionPolicyEnabled, ConsoleSideRetentionPolicyCount, AgentSideRetentionPolicyEnabled, AgentSideRetentionPolicyCount