First off, while there are few outsiders with non-standard alignments, they do in fact exist. One of the Planar books had a city in the maelstrom that was essentially populated with such outsiders - nongood angels, chaotic devils, anyone else.
That being said, the only times the process is described in detail would be Arueshalae from Wrath of the Righteous, but that one got kickstarted by divine interference.
Also, even minor devils are unaffected by age - that imp might be incredibly old, and redemption might take more than a human lifetime.
When it comes down to it - if it's just about turning the imp away from evil, then the most efficient way might be appeal to its lawfulness. Offer it a deal to keep it around, and then try to turn the "tempted to evil by a devil" story on its head.
Another source explaining how at least one devil got redeemed would be "The Redemption Engine" novel, by James L. Sutter.
One of the important character of this story is Arathuziel the Chained, a former deimavigga that, to put it shortly, went through major existential crisis and literally shed its evilness and became a redeemed angel in the process.
Do bear in mind that, to Outsiders, alignment isn't just something they may or may not lean towards. An outsider is an embodiment of something, and devils, daemons and demons are evil by their very nature. So, to redeem themselve is a traumatic event in itself.
But yeah, if you make the imp realize its worldview is wrong (good luck on that), you could end up redeeming it. Not gonna be easy though.