You can't extend the validity.
So if it's a three month pass, then the latest you can travel is 31 August

It is said I have a global pass for 15 days. So.. How to understand its period of validity?

The validity will start on 1st June at the latest.

If you've got a 15 days in a row pass it's valid until 15th June.

If you've got a 15 days within 2 months pass you may use it on 15 days of your choice until 31st July.

If that doesn't work for you, a 85% refund is possible.

Thank you so much. Yes, I have it in a row

The validity will not start automatically.

If you don't activate the pass, and your pass is refundable, you can claim a refund up til 6 months after the last possible activation date. 

https://www.interrail.eu/en/support/already-bought-pass/can-i-return-my-pass-for-a-refund

@Maria Mota You can also exchange the Pass for one you can use in Autumn. However, you will have to pay the full price, but instead of costing you 15% of the Pass from the refund, it will cost only €15 to do the exchange.

Warmly  
St. K

Social Care Specialist

Eurail.com/Interrail.eu

Hello St Kristoffer,

Could you kindly assist me in processing a refund for my ticket? I've attempted to locate the refund option on https://bilet.intercity.pl/lista_biletow.jsp, but I could only find a cancel option, which I've already requested. Unfortunately, it appears that this action may not be appropriate for my situation. When I click on the 'Order details' button, nothing happens, please see the attached screenshot.

Thank you for your assistance.

This looks like a cancelled reservation. If that doesn't work as expected, you need to contact PKP Intercity.