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Is it the DVR that is beeping, or the alarm system itself?
If it is the alarm system, go to its keypad, and enter the 'code' you should have for it.
If you inherited the alarm system, and so you have no 'code', then you cannot reset it and cannot turn Off the alarm. So you might have to disconnect the power for the alarm system and the phone connection for it, too.
If the DVR is beeping, look at the indicators on the DVR. Is it what is beeping? if so, the indicators on the DVR box, might indicate what is wrong. Let me know, please, so that I may advise you further. Thanks.