Formula to change data in adjacent cell

Hi

I have a spread sheet with two columns S and T with 2400 rows

Data in column S is as follows:

Row 1 P

Row 2 P

Row 3 P

Row 4 P

Row 5 S

Row 6 P

Row 7 S

Row 8 P

Row 9 J

Data in Column T is

Row 1 Main Tenant

Row 2 Main Tenant

Row 3  Main Tenant

Row 4  Main Tenant

Row 5 Daughter

Row 6 Main Tenant

Row 7 Daughter

Row 8 Main Tenant

Row 9 Joint Tenant

There is no pattern to what is in each row. When a Joint Tenant appears in a row i need to change the row above from Main Tenant to Joint Tenant

So in the example data above Row 8 Main Tenant should be changed to Joint Tenant

Is there any way this can be done

Thanks in advance