can you be more specific?
when you purchase a security, THAT is the 'cost basis' information for a future sale. If you don't have the entire history of purchase transactions, then maintaining cost basis is problematic.
is this for a non-qual or IRA account? for IRA's, cost basis really doesn't mean anything.
You can do it. But it's not exactly "editing".
There are 2 types of transactions which add shares into an investment account, and they both allow you to specify the cost basis. They are "Buy - Shares Bought" and "Add - Shares Added".
You must locate the transactions that added the shares to your account and input the cost. That will be your basis. You may need to refer to old Fidelity statements to get this information.
Oh, and if you have Placeholders in your account (which is likely if you are missing valid transactions), resolving those placeholders should help get your cost bases into Quicken.
FYI, OP has been discussing this same issue in this post: