If a branch represents a summary task (i.e. a task with sub-tasks) you will find its values are not editable, that is because summary tasks have their values calculated based on their sub-tasks.
As an example, if a summary has two sub-tasks that are linked to start one after the other, each with a duration of 2 days, the summary task will automatically calculate its own duration to be 4 days.
If you consider the map view, when you view your map as a Gantt chart parent branches are converted to summary tasks and branches with no children become tasks. If you switch back to the map view temporarily you should see the dark grey summary icon (downward-pointing shield) against those tasks that you can't edit.
If a summary task represents something that you need control over it should be a task in its own right.