Once imported, the trial balance values appear ‘greyed out’. Click on an entry and a new window opens:

 
Select the button entitled Remove imported balances this does not remove what you have imported, just allows you to edit the values

A new window will open:

Select the top option Remove the mapping information for the Current Year and allow balances to be edited, using the Unlink button:

The values in the trial balance will now be able to be edited.

Below are lists of the accounts available for each type of company.