You need to immediately deactivate your pass. Then you buy a new pass and ask for an exchange. You need to send both the old and new pass number to Customer Support through this form when asking for the exchange.

https://eurail.zendesk.com/hc/en-001/requests/new

Thanks - I have sent a message via the link you provided - I hope it can get resolved easily as I am probably not the only person to have made this error.

Have you sent an exchange request adding both the old pass numbers and the new pass numbers? Then you will have to wait, probably for some weeks, before Customer Support will refund the price, with a deduction for the exchange fee, for the passes that you bought first.

There is a 15days in 2 months pass or a 10days in 2months pass

I sent a request for an exchange

Is it easy to upgrade as this would work?

@TeeBee 

Interrail will not make any change to the passes you already bought. You bought the wrong pass and Interrail gives you the possibility to exchange those passes for other passes, if the conditions of your passes allow that. 

You will first have to buy the new passes you want and then inform Customer Support about the old pass numbers and the new pass numbers. Interrail will then refund the price of your first passes with a deduction of the exchange fee, I think 15 EUR per pass.

The passes have to be deactivated to make an exchange (or refund). 

If the validity of the pass has started, neither an exchange, nor a refund is possible.  

Here's the detailed information about how to make the exchange of passes.

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

As Customer Support currently is overloaded with requests, you will probably have to be patient to get the money for the first passes back.