The
simplest tool, but neither fast nor reliable, is a text editor and your eyeballs and fingertips...
To be more useful... What kind of text file do you have? Is it 7-bit ASCII or UTF-8? Is it html or xhtml? And, most importantly, is the apostrophe character the same as the right quote? If not, then a simple character substitution would work. However, they are usually the same, so something more sophisticated is required.
I use
quoter to change straight quotes to curly quotes in my html, and straight apostrophes to right single quotes. I don't know if it will do single quotes to double quotes, however. I found it with google, and it is supplied as a lex/yacc source, so it could be modified, I suppose.