In short, to allow changes you need to disable support: in Designer mode go to menu Configuration -> Support -> Support options... and in Support Options form click Enable modification and save the configuration. If this will not allow to change enough objects, in the same dialog click Disable support button.
The longer description is that the configuration can have different support options for different parts of it. Some parts are allowed to be changed without of breaking support and some of them must to stay as supplied if the developer thinks so. For more details see 29.2. Configuration Support of 1C:Enterprise. Developer Guide.