You can use the update mechanism to update from earlier builds of 1.2 as well as from version 1.1 (stable). Note that the build is only available in the dev and canary channels, not the beta or stable channels, so if you are not offered the update you need to switch to the canary or dev channels (to change channels, open the preference dialog, go to the Updates category and change the channel setting). 

If you run into problems, be sure to check the Known Issues page which we'll update as necessary.