Just Keep Backup copy of skeleton you want to delete and Know Rclick Delete skeleton it show option to replace skeleton check dropdown menu for which skeleton you want animation to retargeted and hit replace button , warning msg press “Yes”.
Now you can find the new skeleton by Rclick by all animation with File>>selected all.
Save All in Menu UE4 drop down.
If you don’t understand still follow this : [NEW] UE4 Marketplace Animation Packs in UnrealEngine 5 (IK Retargeter) - YouTube
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