How do I change my profile picture?

You can't. Avatars and the editing of nicknames have been disabled. They are making changes to profiles. No one knows when or if they will be enabled again.

Yahoo stopped letting posters change nickname and avatars or pictures on accounts back in January 2015. No word on when we can change them again from Yahoo staff.

https://help.yahoo.com/kb/SLN25611.html?impressions=true