How to manually set the Cost Basis of a coin

Describe the problem:
Wrong Cost Basis on Bitcoin purchase. I sold all my BitCoin on the 5th of April. This left some dust as far as Koinly was concerned that it decided had some HUGE erroneous cost basis > 100Million USD or something to that effect. Unfortunately I dont have a screen grab of it. A few days later I purchased approx 1000USD of BitCoin at a cost basis of $61020 but Koinly has averaged this with its erroneous dust cost basis and come up with a new cost basis of $61633.55. I cant find any way to fix this and now ALL my future Bitcoin transactions will be wildly inaccurate.

Which exchange/wallet is this regarding?
Binance Exchange Wallet

How did you import data into this wallet?
API

What have you tried to fix this so far?
Tried to find a way of manually adjusting the cost basis but with no luck

Fixed this by fudging a fake transaction.

Sorry. This is only as work-around. Still cant find a way of setting the cost-basis legitimately. Any help welcomed.