You can do this in the Field Design View to change them in the geodatabase, then any time you add that feature class to a map it will use the field alias. If you need to change MANY field aliases or domain descriptions, you could consider using the D_Rename table.
If you have pre-existing maps, this change will not automatically apply to the layers in those maps. Instead you must manually alter the Field Alias on each feature/subtype layer, as well as any popups that have overwriten the default alias of the field.
Something like the utility network properties extractor can make this process easier because it allows you to export properties to a file, adjust them, then re-import them. You can also use the Paste Properties command on a layer to paste the field properties between an already configured map and an existing map.
Because of this it is best to get your field aliases figured out in your data model and initial map before you start making too many additional maps.
