If the cost basis is off, the transaction values could be wrong.
If you want to adjust the asset value for whatever reason (e.g. the spread is too high) you need to do this manually for each transaction:
1. Open the Transactions page:
2. Select a transaction
3. click on "edit"
4. click on "edit asset value"
5. Adjust the "purchase value"
6. Adjust the "sale value"
This FAQ article might be helpful for this topic as well: Coin Price/Value Does Not Seem Correct
The Asset values (price at the time of transactions times amount of coins) are only be refreshed once per hour. This could lead to a small spread deviation at crypto to crypto trades. You can reset all asset values later in your account here, then the transactions are calculated with accurate to the minute pricesbut be careful as in this case all manually added/adjusted asset values will be removed as well.
As always when you modify data, we recommend creating a backup beforehand (and regularly as described here: Create backup, restore or export your account data