Q:

Looking for a simple way to manipulate one column of a matrix

Given  
{{a, b}, {c, d}}

I want to get  
{{c, b}, {a, d}}

with one simple expression.  Is this possible?

A:

You can use SubsetMap for this:
l = {{a, b}, {c, d}}

SubsetMap[Reverse, l, {All, 1}]

{{c, b}, {a, d}}

That is, Reverse and replace the list of elements comprised of the 1nth element of All rows of l.
There are bound to be many other solutions.

A:

I suggest making use of function argument destructuring. The required munger can be written directly from the problem statement. No need to dig out arcane knowledge of list manipulation.
mung[{{a_, b_}, {c_, d_}}] := {{c, b}, {a, d}}
m = {{a, b}, {c, d}}; mung[m]

{{c, b}, {a, d}}

A:

Diagonal /@ {Reverse[m], m}

{{c, b}, {a, d}}

Transpose @ MapAt[Reverse, Transpose[m], {1}]

{{c, b}, {a, d}}

