How to clean up Activity logs

I would like to clean up Activity logs of all leads.

【Background】
      I have changed some data values for deleting unnecessary personal infomation for all leads.
      However, these information leave in "Change Data Value"field of activity logs.
      (e.g. city has been changed from ○○ to null)

I understand "Change Data Value" will be deleted after 90 days.
But, if it is possible, I would like to delete logs before the retention period.
So, could you please tell me answer for my question.

Q1, Is there method to delete Activity logs in all leads?

Q2. Is there method to shorten the retention period of "Change Data Value"field?