nems-upgrade automatically upgrades NEMS. It is very easy to use, and is how I am able to offer NEMS as a rolling release during each major release cycle (called NEMS Branch). This way, you do not have to re-download and re-flash NEMS every time I release a new feature.
Generally, bugfixes and minor improvements do not require manual intervention: they are automatically handled by NEMS' fixes.sh, which you do not have to run manually (it automatically runs every day).
By contrast, rolling release upgrades (ie., point release upgrades) require you to manually upgrade with the nems-upgrade command, which requires root access.
In order to run nems-upgrade your NEMS Server must first be initialized.