File permissions in BeOS can currently only be controlled from the command line or via the SetPerms Tracker add-on. However, if you use Pe as your text editor, things get a lot easier.
Open a file which doesn’t have its write bit set, and the small „pencil“ icon on the Pe toolbar will appear with a red slash through it. Just click this icon to make the file editable, make your changes, and click the icon again to restore the original permission settings.
See also: Set file permissions from OpenTracker