Hello & Welcome to Just Answer,
Sorry I didn't see your question earlier and to hear of any trouble with your new device.
If you are referring to changing the "units" displayed by the device (such as kilometers per hour vs. miles per hour), then that can be done in the "Units & Time" settings screen.
To get there, start at the MAIN screen, touch Settings, then Units & Time, and then the Units button, and set the device to your desired setting.
But if you are referring to changing the actual limit(s) that are used for specific roads, then that value can NOT be changed, as it is part of the "data" that is stored in the device. That data is provided by the "data companies" and may change IF the data companies find an "official change" for a particular road, but such a change can often take many months to make it into the device data, which is only updated a few times each year.
As a reference, I have a link to the manual for this system below, which you may not have received with the device. The Unit & Time settings are mentioned on page 17 of that manual.
I hope this helps. Please let me know if you have any more questions.
Thanks, ***** *****