Find the tenant in the tenant list and double click. Under Tenant Group (Code) right hand side. Highlight tenant name and right click.
Choose assign code manual and type in your code.
NOTE: Initially, the first time a tenant is match the original code goes into the Bank Match area and stays there even after you manually assign a new code BUT the Bank Match code will get overwritten when you do your first import bank statement and match to the tenant, whether you located the tenant by the code or by the name. If they had the bank code only showing on the statement then this what will be used to do the matching