Check the audit log in ePO: you will be able to see that a task was modified, who modified it, and when. Unfortunately you can't see what was changed - the logging system does not record that level of detail.
I'm kind of noob with epo. can you tell where should I refer to this log? I mean audit log
Sure Go to Menu / User Management / Audit Log.
You can also write your own queries in the query builder if you are looking for something specific.