When I drag the formula down it updates the cell addresses, but this is not what I want.

I want the cell address ‘C4’ to be updated automatically as I fill down, but I do not want the cell address ‘B2’ to be updated.

To achieve this I add a ‘$’ in front of the parts of the address I don't want to change. In this case I don't want the '2' in the address of cell 'B2' to change, so I add the dollar in front of it 'B$2"

This stops the address updating. You can add the ‘$’ to the row or column or both. I would normally have added it to both:

=$B$2

Then I could have copied the formula anywhere without worrying about either the letter or number.