You can choose if you can.
Just see if you really need it: what's included, how people like it (news, reviews). I've downloaded an immediate security update for my brand new phone but I hesitate about downloading a major system update pending. They promise "efficiency" but I absolutely mind the direction they go about spying after me and making decisions "for my good" as they expect me to be stupid compared to their "smart" software (which I find insulting).
The sole reason I had to get a new phone (well, I love it but my old one would absolutely do had it not stopped working in some places) was a system update I had downloaded carelessly and it was such a failure it killed the whole manufacturer, a world phone making leader before that (Apple's competitor). I don't want that again.
But I reckon I probably can't stick to the system for those many years to come as applications will outdate depending on the system updates. Furthermore, the currently pending update will be succeeded by another, even more doubtful one (and it's only the last I can get, I believe).
I'll update it one of these days. Perhaps: I still can't make myself do it.