Usually there is some sort of "Mode" button that can be pressed multiple times or perhaps a "Clock Set" button that can be held in. You should see it showing the time with the hours flashing. This is called Time Setting mode.
In this mode, there should be a button to move the number up by 1. Sometimes there is also a button to move it down. Because we just moved to Daylight Savings Time in the U.S., you just need to move the hour up by 1. So press that button once.
You may have to press a different button to move to minutes, then seconds, etc. Do that until you are out of Time Setting mode and you are done.
Also, some devices have a special setting to move in and out of daylight savings. If you have that, just turn on DST and the time should change automatically (1 hour later). In the fall, turn the DST setting off and it will go back an hour.
If you are still lost, try searching the web for a user manual for the clock. Use the make and model number of the clock in your search and look for a Users' Guide or Manual. That should have the details for your specific clock.