fix out of tune acoustic guitar with AutoTune?

Just this once, we can't get the guy back here to redo the part for a
month. I've never used AutoTune, I just downloaded the demo. Can
someone please tell us how to use this thing to fix a flat string?
Does it do that? The part is in double drop D but the B string is
flat. I need specific, click here, click there help. It just makes a
warbling sound right now. TIA.