When you want to work on an existing header file directly (as opposed to populating it by using the alias file), use the following steps to open it.
How to open an existing header file
Double-click the Advanced folder.
The header file(s) in your project are displayed (next to any other files that you have created, such as browse sequences, search filter sets, or alias files).
Double-click the header file that you want to open.
The Text Editor opens to the right, with the header file page shown.