"Way around" is may be not the most approriate description, since you seem to be inventing your rules. And the whole idea looks funny. Do you really have umpteen colunms with Manufacturer and Product names? That looks like an incorrect table design to me.

You could do:

UPDATE tbl

SET col1 = CASE WHEN @columnname1 = 'col1'

THEN dbref.Amended_SW_Manufacturer

WHEN @columnname2 = 'col2'

THEN dbref.Amended_Product_Name

END,

col2 = CASE WHEN ...

But I will have to admit that this looks highly suspicious.