Is it OK to update to the lastest version over an older one
or best to delete first
If it's a minor version update, ie 1.6.0 to 1.6.2 then it should present no problems updating over the top of the installed version, however if updating from say 1.5 or 1.4 I would recommend uninstalling the original version completely first, and in that event:-
Undo the immunisations, disable teatimer and the resident browser helper
Uninstall the old version.
Install the new version, don't enable either the resident browser or teatimer, and don't download the detection updates until the program has finished installing. Once it has, reboot, then get the updates, do a full scan, fix anything found that is shown in red, then immunise, enable the browser and optionally enable teatimer.
To be honest, it is so simple to install and setup that I always uninstall the older version first, but many don't bother.