I just hosed my Centos install and I'm wondering whether to do a
yum update? I have loads of mysql and solr databases that I am working on. Will it delete those? Should I do a backup before doing a
Yes making a backup is ALWAYS the right answer.
No using package managers like yum to upgrade your system will not mess with your data.
The only caveat is that sometimes data formats change between versions of software. For example the data storage format mysql uses might change, and the upgrade scripts might help you convert your data to the new system, but it is very unlikely that a package upgrade like this will actually break your data. Also backups are the right thing to do because if anything goes wrong with system updates such as an unbootable system on a kernel upgrade, that backup will be nice to have in your pocket.