This question seems to come up weekly, here's a handy selection of previous posts that may have the answers you're looking for.

• When did Americans lose their British accents? Also how long did we go with American Presidents that had the accents? Who was the first one who didn't?
• Is there any truth to the narrative that the Southern American English dialect resembles a standard accent from 18th Century England?
• Considering the British colonization of the Americas happened fairly recently, when did the American accent diverge from the British accent and how did it occur so rapidly?
• When did the American Colonies lose their “British accents” and how was this accent different from the accent we hear today?
• How did America (not including the south) get the accent it has today?
• How long did it take for Americans to lose their English accent, and was it maintained while the English still occupied the colonies?
• At what point did the accent of American settlers become distinguishable from a British accent?
• How long did it take for Americans to loose(sic) their British accents?
• When did Americans lose their British accents?
• When did Americans lose the "British accent"???
• I've read multiple articles that state the American english accent is actually what proper English sounded like, pre the America's. Apparently it's the British accent that has changed. How true is this?