How to upgrade between Mint versions is explained at great length by Mint's lead developer here. I recommend you read the entire post, everything you need to know is explained very clearly there.
The gist of it is that you can either do an apt-based upgrade or a fresh install upgrade. The apt based way is simpler and easier but much more dangerous. You can end up with a borked system because of software and library version incompatibilities.
The recommended way is to backup and reinstall. Mint has a number of tools designed to make this process as easy and painless for a new user as possible. The basic steps are:
- Making a backup of the data
- Making a backup of the software selection
- Performing a fresh installation using the liveCD of the new release
- Restoring the data
- Restoring the software selection
If you have your $HOME directory on a separate partition and all your data there, you don't even need to backup your data. You can just backup the installed software selection, install the new version using the same user name and $HOME and then restore your software selection. Just make sure you use the right partition and don't format it instead.
You don't need an antivirus for Linux. There are actually a few viruses out there that can infect a Linux box (wikipedia lists 16) but they are still both few and relatively harmless. There are various Linux antivirus software tools available, the wikipedia page I linked to lists a few. Mostly they are designed to be run on Linux servers in order to protect windows based computers. Seriously though, you don't need one. I have been using Linux for more than a decade and I have never once needed one. I don't think I've ever met a Linux sysadmin who tested her systems for viruses. The problems with Office have nothing to do with a virus at all. Try posting a new question about it and see what SU has to say.
For more information on Linux viruses see here, here and here.
NOTE: While not, strictly speaking, necessary for a new install, having a backup is always a good idea especially when installing a new system. It is very easy to mix up the partitions and use your
/home partition as