You can renew online, if your subscription is up soon, there should be a link somewhere which will take you to the Symantec website and you just put in your card details as with ordering anything else online.
There is no new software to install, what you are renewing is your subscription to download updates. If you didn't renew, the programme would still work after your subscription is up, but you wouldn't get the updates for new viruses etc, so it would steadily become more useless.
The cost will be cheaper to renew than to buy a new updated version in the shops. I don't know to be honest whether an updated version offers any advantages, someone with the newest version may be able to answer this.
There is no new software to install, what you are renewing is your subscription to download updates. If you didn't renew, the programme would still work after your subscription is up, but you wouldn't get the updates for new viruses etc, so it would steadily become more useless.
The cost will be cheaper to renew than to buy a new updated version in the shops. I don't know to be honest whether an updated version offers any advantages, someone with the newest version may be able to answer this.