If you need to re assign accounts to a new account manager this can be done by using either a quick search on the account manager field or alternatively you can find all accounts by selecting the account manager in the Quick List section.

Once the data is retrieved you need to right click on one of the records chose either Select All or the accounts that require to be transferred and then right click again to choose the option Assign Values to the selected.

You can use  the Ctrl and Shift Select options


In this screen you can choose the option Account Manager and then select the account manager required from the drop down list and press Save.