ken1:
I would like to upgrade my Zen Cart from v1.5.1 to the new 1.5.4.
I am using the template Sheffield Blue. If I upgrade does my template
stay intact.
As long as the template didn't require any changes to the ZenCart 'core' files (those that are a part of the ZenCart distribution files), and as long as all of the template files are in their own folders, then yes, they will stay intact.
FWIW, although it is generally recommended to compare the files for any relevant changes, I generally find it quicker and easier to forgo this step (especially for template related files) and just go ahead with the upgrade, and if this does happen to 'break' things I'll just re-install the template according to the original instructions.
You should keep a full back of the site before doing this though, because there are often(?) times when the template files have had their own modifications for any given store, and performing a reinstall of the template will negate those changes.
Cheers
RodG