Do I have the right to change flights when the airline have changed the time?

I booked flights for the summer and my return flight has changed time, and is now earlier. There's a stop over, which would now be a very long wait due to the first flight being earlier. Will I be allowed to change the second flight to a earlier one, so I don't have to wait so long?
Also who do I change this with if I do, the company I bought the tickets from or the airline itself?

Call the airline. When an airline I have a frequent flyer account with changes the time of a flight, they send me an email. They say that I may be able to change my flight with no additional charge. So call and ask them.

If the itinerary has been changed, they would almost certainly looked at whether an earlier connection could be made, the earlier flights maybe fully booked or simply none available.

You work with the airline and not the travel agent. You do not have any rights in this, but a good airline will try to satisfy their customers if you call into their service center. They may waive a change of flight fee. They may waive a fare difference. They may offer a meal voucher for the airport or on the next flight if not changing the ticket for you. You have to nicely beg complaining about new arrangements to the airport and the wait in the terminal between the flights. They will not make a major offer unless the time involved is extra long as in over two hours. Definitely, some airlines are better than others in their responsiveness and call immediately after looking up potential flights from the same airline.

YOU DO NOT HAVE THE RIGHT to just take any flight you want.
YOU ASK if there are seats available on your desired alternate.
MOST Airlines will not charge a fee for a minor change in a situation like this. SOME AIRLINES DO.

Best to deal direct with the Airline. They control the seat NOT SOME TICKET SELLING SITE.

You don't have the right however you generally are allowed to ask for other options when the airline has changed your flight ahead of time. But you need to contact the airline.

Buried deep in the contract of carriage you didn't read is a provision that states they can modify your flights and connection cities without notice and recourse provided the departure and or arrival occur on the booked day.

No.