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First, lets see default 'Post Revisions' in WordPress
There is only 'restore' option in default Revisions feature
Goto Plugins > Add New
Type 'Wp Revision Request' & click Search
Install & activate this plugin
Open post in 'edit' mode
Now 'Post Revision' also have 'delete' option
Click 'Delete' to remove specific Post Revision instance
There is also option to 'create new revision' point on demand
Comparing default & new 'Post Revision' options
Deleting post revision instances in now easy - isn't it?