WordPress does not provide an easy way to delete old revisions. One solution is to the use the PublishPress Revisions plugin.
When you are looking at revisions for a post, PublishPress Revisions will add a “Manage” link at the top of the screen.
On this screen, you can browse through all the revisions for this post. You can click “Delete” to remove single revisions, or you can use the “Bulk Actions” feature to delete many revisions at once.
Please note that you won't be able to delete revisions that are waiting for approval. To do that, go to the “Revision Queue” screen.