Yes, you can modify the amount invested by an investor.
In order to do so, you can:
Go to your deal's page
Click on the commitment tab
Select the commitment that you would like to edit
Click on "Amount", and enter the new value
You can only edit commitments after your investor has committed, which means that you can only edit a commitment that is "Pending approval" or "Approved commitment".
You cannot therefore modify the amount yet to be committed by a user still in the "Requested Access" or "Awaiting answers" buckets.