Opting of regular taxpayer from composition scheme during the Financial year-reg

sir,
one of my friend has opted regular taxpayer from January 2020 from composition scheme, however, he got realised that he has erroneously changed the pattern of composition scheme to regular taxpayer.
he has availed the ITC as well as his buyer have also taken ITC.

what is the procedure for rectifying the said falt.
pls explain along with relevant sections and rules.

thanks