Hello Javed -
Do you have iOS 18 now? Or 18.0, not sure how it is indicated. If so, this is not a large update, and shouldn't take much time. If you are updating from an older iOS, the notes below might be helpful.
Always a good idea to back up your iPhone before updating.
How to back up your iPhone, iPad, and iPod touch - Apple Support
Make sure you have a secure, reasonably fast wifi connection. 25 megabits per second is probably enough.
As the 11 is one of the older phones compatible with iOS 18, it can take quite awhile to install, maybe a couple of hours, so have it charging.
This may make the phone heat up, so take off the case and make sure it's well ventilated.
After the new operating system is installed, your phone will need to index the items on it (pictures, apps, data, etc.). So the phone may use up a lot of battery for the first day or so. This is normal.
Beyond this, it's hard to know what may crop up. Many issues clear up if you restart the phone once or twice.
Restart your iPhone - Apple Support
If you have any problems that you can't figure out, come back here and describe them carefully. But please note that if your question is not very similar to an existing one, it's best to start a new query.