[erlang-questions] Modify a list of tuples

[Dave Bryson]

What's the best way to modify a value for a given tuple in the list  
below?

I have a large list with the following type of structure:

[ [{a,1},{b,2}], [{b,3},{a,4}] ]

I want to preserve the structure of the list and change a value on all  
the tuples with a given key. So for example, change all the values on  
the "b" tuple so the result would be:

[ [{a,1},{b,CHANGED}], [{b,CHANGED},{a,4}] ]

I've tried list comprehension but can't get it to preserve the rest of  
the list.

Thanks in advance!

Dave