Refresh Matrix with data updated

1.- How Can I Active a cell that it's inactive So I update that cell with a new data?

2.- Besides, Is it possible in SAP BUSINESS ONE to change a data that it`s located in a locked cell for the Standard? Is There any command so I can active that cell And I can update with any data?

3.- What does "Item is not an user defined item" mean?

Thank you for your help.