There is noone in the Community who can help you with this, but we can tell you how to do. 

First you need to check if the conditions for your pass allows an exchange of passes. You find the conditions on the invoice that came with your confirmation email. You also find the invoice in the order overview in your Interrail/Eurail account. 

To make an exchange of passes the old pass has to be unused and deactivated. You must purchase the new pass and then you send both pass numbers to Customer Support. You will then, after a week or more, get a refund of the price for the old pass less 15 EUR. If you bought PLUS you get a refund of the whole price. 

Here you can read the exact details.

https://www.interrail.eu/en/interrail-passes/ordering-info/refund-exchange-policy