If you're running REAPER, any bit of a track can be edited by increments of 1/100th of a semitone. Several years ago, I recorded an excellent pianist who wanted to sing three-part harmony on some of the tracks. There were places where the pitch had to be pulled up by 30 cents(30/100th semitone). All was successful in the end.
IMHO, I'd avoid any type of auto-tune. The timbre suffers.
I think I would re-sing in it, and try to get it right. And skip the Auto-tune entire. I've re-sung a lot of stuff until I got a take I could be happy with. Using Auto-tune is anathema to me on vocals. (I don't have Antares Auto-Tune, but I do have Melodyne Studio 4.) I have three singers I'm recording right now, and last week one of them sang flat on a verse. I pointed this out, and she re-sang it, and nailed it. I tell them that I don't tune vocals, except under extreme circumstances. All three singers are good, and they will only get better if they strive to get it right. Auto-tune is used almost universally at this point, even on singers who don't need it, or shoudn't need it (like Adele). It's just a weird modern thing.
I ain't sayin' don't use it. I'm just saying try to get it right without it.
+1 to the advice given.
In order of preference I have the singer take another go at it - even if it's just a small passage for one duff note.
If that's not practical then I'll raise, or lower, the specific part with ProTools built in pitch adjustment, which sounds similar to what dfacker is describing in REAPER.
Lastly, I'll use auto-tune software, but with manual adjustments.
I try to save the last option for people who aren't really great singers, but I want to make them sound as good as possible.
I only record myself. If I wander on one or two words slightly I use ReaTune (Reaper) to manually correct. Usually that works very well. If I'm sort of "sliding" up or down into a word....then that is far more difficult to correct without it being more obvious. Longer errors....I re-sing it.
As for re-singing a line.....I do that in real time if I realize I messed up. I just do another track and on many songs it's easy enough to pick and choose the best lines...words....etc. I've had as many as 10 tracks of takes I've used to cut and paste my best vocals.
On that same thought......re-singing a line later.....like the next day or so.....always seems to sound different enough for it not to work well. Same setup.....etc...etc. My voice is just not 100% the same as the original takes. Close...but I notice the difference.
2 cents worth of.......who cares how I sound......oh me.
To all those who gave me the advice I asked for many thanks..Re recording would be awkward for just a small part where Im slightly flat so I will try a couple of things thats mentioned re auto tuning that one part Cheers again Frankie