There are a lot of components that go into accents, ranging from how individual phonemes are articulated to sentence-level pitch contours. You can't hope to convert an accent into another one without an extremely laborious manual process that involves carefully adjusting formants and pitches. Even swapping one phoneme for another sounds tough.
I'm not aware of any software that could do this, either. Praat is mostly analysis and visualization, and I don't think Melodyne can control formants to that precision.
It's actually a pretty interesting potential research topic, but there don't appear to be any solutions available today.