Hi eaes,
You can change UPN from the on-prem AD directly, and then run dirsync to force sync the change.
Moreover, if you want to change email address, you can use ADSI Edit to change its email address.
1. Open ADSI Edit.
2. Click Users and right click the user who you want to edit.
3. Double click the proxyAddress attribute.
4. Add the address as the format below:
SMTP:abc@xxx.com
Note: Be sure to use uppercase "SMTP".
5. Run dirsync to force sync the change.
Note: The change will take some times to take effect.
If you need further assistance, please feel free to contact me.
Thanks,
Jerry Cen