You will have to give more information to get a useful answer.

If you have activated your pass and the validity has started, then it is normally not possible. But you need to give more information so that we understand exactly what you want to do.

Good morning, thank you for the reply.

so I have an interrail France pass, a flexi pass of 8 days. I needed to activated before 6th of April but i would like to do my trip in July in order to spend more time in the country . I was looking for a way to change the start date. 

You can’t extend the validity of the pass, however in most cases you can exchange it for a new pass for a fixed fee of €15, see this page for details https://www.interrail.eu/en/interrail-passes/ordering-info/refund-exchange-policy

The procedure is to buy a new pass then apply for a refund of the original (minus €15) but please read the terms carefully to make sure you are able to get this exchange with your pass.

According to the information from Interrail/Eurail the pass must be activated no later than exactly 11 months after the purchase date. What you can do is an exchange of passes, if the conditions of your pass allows that. The conditions of your pass is on your invoice. 

If you can make a pass exchange you buy a new pass and then you will get the price for the old pass less 15 EUR refunded. If you bought PLUS then you will get a 100% refund.

I wonder if you bought the pass during the 50% sale? If so most of these had limited possibilities so check your original e-mail with the pass details on it.

1 country 8in1month pass was not offered in that sale.

Also as an aside as so many questions about this come on the forum; you canNOT use that pass for the trip to/fro FR in TrenItalia-have to buy normal other biglietti for that. 1 country means just what it says: solo uno paese!