Asus has released the Marshmallow OTA for the Asus Zenfone Selfie that was released back in August 2015. The update comes as an OTA and firmware ZIP with software version 18.104.22.1682. Right now, the Android 6.0 update is available only for model no. ZD551KL.
Hop into Settings and then into About device section to look for the system update. if your device is eligible, then you may be rocking the Android 6.0 update on your Zenfone Selfie in next hour.
However if OTA doesn’t get you Marshmallow love, then go for the firmware download. But, before downloading, make sure that your device’s region is WW, because as you can see in the software version’s label — WW_22.214.171.1242 — the Marshmallow firmware is compatible only with WW Zenfone Selfie.
Download Zenfone Selfie Marshmallow firmware: version WW-126.96.36.1992
If you need help with installation of firmware on Zenfone Selfie, then look at the guide available here.
→ Be sure to created appropriate backups first, though. Asus Marshmallow update is know for creating issues more than often.
Check the full changelog below.