You might need to provide a little more information such as Lightroom version CC 2015 or LR6, what operating system and have you a current subscription (if you have CC) or a serial if you have LR6.
Note that there is no unpaid upgrade from 5.7.1 to LR6 but if you are using CC you should automatically have access to the DeHaze function in Lightroom CC 2015.1
And if it is CC just log out from the creative cloud and log back in again.
Also, take note that the Dehaze function is not available for Lightroom 6. If you have LR 6 then you will have to wait for LR 7 in order to get the new features.