Downgrade from 2.70 to 2.64

Well, I wanted to stay in 2.64 so I turned automatic updates off. When I connected my watch to Garmin Express, it downloaded the new firmware. Even though, I didn´t update the watch. This morning, when I pick an activity (walk) in order to obtain a lecture for the gps, a message appeared in my watch: firmware was updated to 2.70.  Why?! I didn´t want this update! I just wanted to stay in 2.64 because I wanted the OHR for swimming! 

The question is: how can I downgrade to the 2.64 beta? Is there something I can do, and don´t mess the watch?

Thanks in advance!