i am asking a question where once we make a feature class and we want to change the field alias because when we first made it we had underscores. we are trying to get rid of the underscores within the fields tab it allows us to remove it and we can save it but once we bring int he layer in a new map the underscores are still there. does anyone know why this is?
try Alter Field (Data Management)—ArcGIS Pro | Documentation
If you make the change to the field in the map layer, it will only persist if you copy that layer to a new map. You haven't made a change to the feature class itself, just the layer.
However, if you change the field alias for the feature class in the geodatabase itself, then it will persist for any time you add that feature class as a layer to a map. Go into the Catalog view, open the geodatabase and right click on the layer. Select Data Design | Fields and alter the alias of your field.
To Ken's point, you have to modify the Data Source rather than the Alias for the layer. To do this, open the Fields Designer, at the top left where it says "Current Layer", click the dropdown and select "Data Source." Then modify the Alias name. When you've updated all your Alias'es, click the Save button on the Fields Tab. The Data Source change is complete and will be reflected in all new Map Views and Projects.