Open this link: https://myactivity.google.com/page?hl=en&page=youtube_likes
It will bring you to the Youtube likes and dislikes webpage.
Then just click the “X” button beside the Date, e.g. July 21st.
When you do this first time, Google will pop up a dialog for notice. Then you will find you can delete without any pop ups. So you can quickly delete likes and dislikes by click the “X” button one by one.
If you want to delete all records, just click “Delete All” from the left panel:
Then it will pop out a dialogue like below:
After deleting, you can go to https://www.youtube.com/playlist?list=LL to check if the action effects. Remember, it will take several minutes to effect on Youtube webpages, just wait for a while.