Never Ever do anything with an operating System without backing up first.
While it is usual for a new OS to install with no loss of data, the time you do it without first backing up will most likely be the time that you wish you had backed up.
In all of the updates and upgrades Apple distributes there is a recommendation to backup your data before installing.
The easiest option is to get an external hdd if you do not already have one, plug it into the mac and follow the prompts to use the drive for Time Machine. Note that any data existing on the external hdd will be erased so use a new one or one that you are confident is ok to erase without a disaster ensuing.
Also ensure that System Preferences -> Energy Saver has been set so the Computer Sleep is Never so that the backup will not be interrupted if this is the first backup or it is a long time since the last backup was done.