Are you sure you want to remove the historical data that goes to the old GL#. You can just make that Acct Inactive and then just create a new one with the new number.

But if changing the Acct# is what works best for you...

Go into your Chart of Accounts and in the Drop Down menu at the end of the Acct line pick Edit.  You can modify all of the Acct info there.  You may want to make a note of the old Acct# just for future reference.