From the ClamAV website page on how to upgrade:
If you installed from a package, we suggest you find the approved package from your distro provider and install that. The ClamAV team does not maintain individual packages for every distribution build. If
there are no new packages, you have three options:
- Wait
- Build Clam Package
- Install From Source
It is OK to wait because clamav is still working even though it's not the latest version. That warning is more FYI. Also, you don't need to manually run freshclam. If you type
tail -20 /var/log/clamav/freshclam.log
you should see some more info, telling you that the databases etc have been recently (as in, today) updated. When I do this, I see the same warning as you and the next message is don't panic: see https://github.com/vrtadmin/clamav-faq FAQ page
So, don't panic.
I do not recommend compiling from source, as it will not be updated through APT, but here is the guide
Summary:
Uninstall the old version sudo apt remove clamav
Download the tar file from the ClamAV website
extract with tar xf clamav*
then cd clamav*
and then:
./configure
make
sudo make install
then run
sudo freshclam
As always, see the documentation for more help