Hi Scott,
there are two ways you can do this. A temporary way and a more permanent way.
the temporary way is using symbol substitution. this change does not edit your data. it only exists for the project.
right click on your anno in the contents pane and select Symbology. this will open the Symbology pane. In the Symbol Substitution group select the Substitute symbols in collection radio button and double click on the symbol. Change the symbol as you wish from here.
the more permanent way is to edit the symbol in the annotation feature class.
In the catalog pane browse to your annotation feature class. right click on it and select Annotation feature class properties. This opens up the Annotation Feature Class Properties pane.
If you have feature linked annotation and want the symbol change to be reflected in updates you need to click on the Annotation Classes tab, double click on the annotation subclass you are looking to edit and then select the symbol tab. Make your symbol changes here. Now for all newly created annotation in this subclass, the new symbol will be used. To change the existing features, you'll need to edit the symbol id field to be the newly created symbol id from the collection.
If you have annotation that isn't feature linked you can either create a new symbol in the collection by clicking the plus sign or double click on a symbol to use that as a starting point. Once symbol edits are made and the new symbol is created, just edit the symbol id of the features to point to the newly created symbol.
thanks
Wendy