The updates are sent to the networks to ensure they are compatible, networks require different parameters, sometimes, if it’s ready you will get a notification.
The check for updates isn’t continuous so you may have to wait for your phone to ask the network also.
The update is an OTA (Over The Air) notification via the network and then downloaded via your data allowance or preferably your wifi connection.
It’s pretty automatic > Download 4 min > Installation 15 min > restart > 1 min etc.
You can ask for the update via Settings > System >System update
If you don’t see one that means it is not available for your network yet.
The alternative is to download the new OS and install manually, which is good if you have a real problem with an existing version, but not ideal. a) it takes a bit of effort and b) it may not work for your network flawlessly.
Check out the following post as some people inform the forum when their network has provided the update.
See other instances of delays