Here are some useful tips on how to retrieve your lost contact when you upgrade your infinix hot 2 from lollipop to marshmallow.

Goto to contact at your menu bar.
At the top right hand side of your contact select import/export.
Select the sim you lost your contact e.g IMPORT FROM MTN NG.
All your contacts will appear on the screen then select import all at the top right hand side of your phone screen.
At this point all your contact will be back again'

I hope this will help those who are complaining of losing there contact when the upgrade their android phones.