Had these two flights been on the same ticket you would have been able to rebook for free.

Unfortunately this is one of the risks with separate tickets, you are not protected in the event of delays or advance schedule changes like this which are not classed significant (ie. 2+ hours). So it is always wise to build in a comfortable gap between flights on separate bookings to give a cushion against delays and changes.

You can still change one or both of these tickets of course but it won't be free.

Which flight it is more economical to change, or whether you are perhaps better off cancelling both and booking a new combined connecting ticket, will depend on the fare rules of what have already booked and current fares.