i just got this same pop up, clicking yes i am currently on hold to see what it is. but i have the assumption nothing takes place because they are sending an update but one we already have on the phone.
however theres another poster who mentioned that after clicking yes "unknown sources" appeared.
but over the last week+ ive gotten the pop up about 4-5 times and nothing on my end has changed.
the response of the manager im talking to is this is an "htc" issue, but hes off trying to find out more.
i think id like this tech support more if i could call back the same tech instead of starting from scratch every-time i call lol