Remove the formula for the line item, but I am not sure you want to do that as the formula was put there for a reason, I would think.
If there is not a formula, then you are viewing data at an aggregated level (detail time granularity is at days or weeks while you are viewing/showing months or list is at the SKU level and you are looking at the Brand level) and there is nothing to be done, unless you create another module at the summary level you are desiring.
There are several reasons why data can be closed from editing:
1. They are calculated using the formula (as Rob wrote above). 2. This is aggregated data. 3. You do not have write access to the cell (governed by selective access, version settings, or dynamic cell access). 4. These are the actual months on the forecast version.
To check the last point: see if there is a version in the module and if the switchover on. It is possible that you do not need it.