Previous years closing journal entries made to increase A/R, were done in error. Because the company is on cash basis, payments received were credited to the income G/L line, so the A/R line didn’t decrease. Due to unorganized bookkeeping, I am not sure how to reverse the journal entry. What G/L line should be debited in order to decrease the A/R line.

If you are going to reverse the adjusting entry you do exactly that using the same accounts