Dynamically update a table column with a single value from another non-related table

Table A has multiple rows with a value that needs to be updated based on a single value in another table

Table A has 3 columns

Column A is a money value (£). Many rows

Column B has a factor that is stored on a different table

Column C is a calculated field colA x colB

How do I update column B with a factor, which will change regularly over time anwhich is held in an unrelated table