You're best bet is setup a Central to scan for advertising devices which have old version of code. If so update, then continue scanning for old version. Helps to have the Version inside the advertising data.
Is this after the devices has been shipped to customers or in production? If its the latter, then I would download pc-nrfutil, found here, and it to perform the DFU of all your devices, one by one. You will have to write a small script where you have a list of the MAC addresses of the devices that you want to update and then call nrfutil with the DFU option for each one.
We have a lot of nRF52 based products without the JTAG connection, and would like to do a batch update. We can do OTA updates serially by pairing. I was wondering if it was possible to do multiple devices simultaneously?
