There are two ways to change this in 11.0 and higher:
To change an individual alert (Or in versions prior to 11.0): Go to log into the server that contains your Orion platform, go to Advance Alerts, click on the Alert you wish to change. From there go to Trigger Actions, select the send email/page action. On the first tab to open, it has a default from address that it sends from. Change this to what your selected address is and save. Then ensure to change the Reset as well.
To change all alerts (on all versions 11.0 and later): Log into your Web Console with an Admin account. Go to settings, and under the "Reports and Alerts" section, go to Initial alert setup. There you can specify the default to and from for all alerts. This will also setup a default setting for all alerts you create. Ensure you have only alerts enabled you wish to use if you go this route, Otherwise anyone on that default to and from, will get every yet to be configured alert that has a default send email/page action.
Hope this helps some.