Hi Heather,
I understand you deployed Dirsync to sync on premise users to Office 365, and would like to let users change the password.
For these synced users, they can change the password in the computer(CTRL+ALT+DEL from their machine), so that the Dirsync can sync the changed password to Office 365. As a tip, when you set up Dirsync configuration wizard, please select enable password sync feature. If the users have not company computers, they cannot change the password, and need to involve Office 365 administrator to help change the password in AD.
If you have not Dirsync+SSO environment now, and the issue occurs, please let me know. We need to dig deeper on the issue.
Thanks,
Johnny Zhang