Hi you have to go to trains staff and they have to book for you for free a new reservation. But please keep in mind that if the other trains are full, you will stuck. In France you can ask directly in so a situation train staff of the next train, if you can board without reservation. In such a case, they often let people stand in the train without reservation. 

You can also get money back for the delay (if come to your destination with more than 60 or 120 minutes you get money back from Interrail, but also if you don’t use the reservations)

Sometimes there are problems with this rules, when from one train to the other you have 2 different train companies. 

There are EU-rules for that and thus tehse only work IN the EU. TCDD in TR will not do it.

DO be very careful exp for DE=Germany-, where I am now., Here well over half of all trains are so enormous late (today I saw like 80, 90, 110 mins) or even cancelled, that any plan goes down the gutter.

The original idea of the pass -and most people still use it like that-to do 1 rather slower and sight-see trip from A to B,stay a few days to see, then next trip-not for enormous long trips with plenty of very sharp connections=garantee it will go wrong