How to update a table based on values in another table?

I have 2 tables. I need to update some of the values in table A based on values in table B.  This is part of a bigger function.

Table A has two columns. Source and Tactic.

Table B has one colum. Source

If Table A source = one of the values in Table B, then Tactic should be X else Y.

How to do this? Do I need to iterate through all the values in Table B each time? There are many thousands of rows. 

Note: Tables A and B are updated independantly.

Thanks for your help.