How can I change a field in a SQL server database that's set to "Read Only Cell"?

What is the datatype of the field? You may not be able to "type" into it if its of an ntext or image datatype and management studio can't handle the size of it.

In that case you might have no option but to perform an update as follows.

UPDATE TableName SET ColumnName = 'NewValue' WHERE PrimaryKeyId = PrimaryKeyValue

The field is most likely "read-only" because it contains a calculated value.

If that's the case, you would have to change calculation in the table definition to change it's value.