How to bring back a Time Machine backup to only the latest file versions?
Start by mounting your Time Machine backup disk. Then open Terminal.app and issue the command:

tmutil listbackups


It should give you a list with dates and time for all the backups stored on your drive.

To delete one of these backups, copy the line outputted by the listbackups command and append it to this command:

sudo tmutil delete


So it will look like this for example:

sudo tmutil delete '/Volumes/MyDisk/Backups.backupdb/MyMac/2016-11-24-003000'


Make sure to delete only the older backups leaving the newest or some newer backups on the drive.

The last step is to compact the sparse bundle to release the now freed space for other purposes:

sudo hdiutil compact /Volumes/MyDisk/MyBackup.sparsebundle


(remember to edit the path to match your setup)

It will output status while compacting the bundle, and eventually tell you how many GB it freed.