Cost basis of a disposed asset is calculated automatically by Koinly based on your previous transaction history and cost basis calculation method selected. It cannot be edited manually.
As such - cost basis of the asset upon disposal (sale) is already determined - you cannot change it. What you can review (and possibly adjust) are the transactions that led to this disposal.
Check:
Which cost basis calculation method am I using? How does it choose which lot to sell?
Are there any special rules in my country that may affect the calculation (Cryptocurrency Tax Guides)
Are there any errors in my transaction history of this asset? Warnings like Missing purchase history and Market prices for XYZ are missing will affect the calculations
Is my transaction history complete? See How to ensure your Tax Report is accurate - especially the part about unmatched transfers
How/when was this asset acquired? "Cost Analysis" tab on the transaction will help here, especially for FIFO/LIFO/HIFO
Refer to Cost basis shown on my transaction is wrong for more tips on how to assess the cost basis - in most cases, if the transaction history is complete, it turns out that cost basis is calculated correctly, even though it's different from what one was expecting.