Delete a file extension from backup

Hey guys.
I am a newbie here. My first full backup isn’t over yet, but I made a mistake and I don’t select the file extensions to exclude.
How can I delete these files? I have now correctly placed the extensions to exclude, but will this only work for new backups?

No one can help me?

Yes It will only work for new backup
Why not waiting for default cleanup nightly done ?

That’s what I would like to know. If I update the list of excluded extensions, will all these files eventually be excluded from backups already performed?
Because I backed up hundreds of gigabytes of music and videos that I don’t need only after I set to exclude those extensions.
Just wait or is it better to delete all downloads?