I'd suggest you read the DesignBuilder LEED and ASHRAE 90.1 User Guide here, which covers both setting up a 90.1 model from scratch and also converting an existing model: https://designbuilder.co.uk/helpv6.0/#ASHRAE90.1Modelling.htm?Highlight=leed

If you still can't figure it out then I'd suggest raising a ticket on our support desk.