In most of the cases it should be enough to edit source code in places which are not in the blue guarded blocks. If you'd like to edit code in blue guarded blocks it's possible to do it through Custom Property Editors window - Custom Code form. Source code locked in guarded blocks MUST NOT be edited directly.
NOT RECOMMENDED SOLUTION: To remove all guarded blocks from source code it's necessary to delete *.vmd file linked with Visual Midlet/Visual Design class. The Visual Midlet/Visual Design class without *.vmd configuration file should be visible in Netbeans Java ME project as regular Java Class and entire source code should be editable. In this case all additional information about Visual Midlet/Visual Design are irretrievably lost (except source code).