I know this is old but I had a similar need for this and I did not want to do the find and replace version. It turns out that you can nest the substitute method like so:
=SUBSTITUTE(SUBSTITUTE(F149, "a", " AM"), "p", " PM")
In my case, I am using excel to view a DBF file and however it was populated has times like this:
9:16a
2:22p
So I just made a new column and put that formula in it to convert it to the excel time format.