Upgrades

that's the new version of zen cart out on official release now, thats why you got that message, if you sign into the admin of your zen carts, at the top right hand corner, it should have a prompt there aswell for the new version.

Dont just upgrade the zen cart, first you need to check what mods you have installed on your site and see if there are updated versions of the mods you need.
There will be quite a few mods that wont work on the new version unless the mods are updated, so dont try and use any mods unless they have a version suited to 1.5.0 (it will say in the read me files for the mods)

If there are mods that you need and they havent been updated for the new version yet, best waiting a few weeks to see if an upgrade becomes available.

before you start the install, take a copy of your database,

log into cpanel, click on the phpmyadmin icon and on the new page, click on your zencart database in the left top corner, that will display the contents of the database, then on the header menu, click export, and save the file to your computer as a .sql

if things go wrong when you're doing the install, use the R1soft system in Cpanel to revert all your site files back to the way they were before you attempted the install, and then go into phpmyadmin and import the .sql file you saved to your computer (sometimes its easier to empty the database totally before importing the .sql file you saved)
that should revert your whole site back to the way it was, just incase problems arise, which they might do, there are quite alot of changes in the new version of zen cart

there is an easy way to do the upgrade, which doesnt really involve picking through loads of individual files, take a copy of your template, create a list of the mods you've already got installed and find the new updated versions of the mods.
Then when you have your template and the updated mods, delete the whole zen cart out (keeping the database) upload the new version in its place, run the install as normal but choose upgrade database (not install) and then install your template and mods, job done.

Aslong as you keep the original database, you wont loose any important data (orders, customer info etc)