Updating your Raspberry Pi frequently is not only recommended but also important for multiple reasons, the biggest one being security. Like any other compute device, Raspberry Pi is also open to potential vulnerabilities that requires contant detection and fixed. The developers that maintain the Raspian OS continously update the fix newer vulnerabilties.
This is especially true if your Raspberry Pi is internet facing, i.e. it is being used as an

How to update Raspberry Pi
1. First update the repositories by running
sudo aptitude update -y
2. Then upgrade to the latest version of the distro by running
sudo aptitude upgrade -y
3. FInally if there is a new distribution (major new version), you can upgrade to it by running
sudo aptitude dist-upgrade -y
That’s it, these three steps keeps your Raspian OS upto date