I have run into several issues with paths during installation of Sitecore Commerce 9 Initial Release.
I have tweaked couple of paths so they are not ambiguous or wrong in initial ps1 script. Here
Here is my script which is for Commerce 9 Initial Release but should be more or less same also for Update 1 (use WinMerge to compare ps1 files):
Here is also complete guide on how to install Sitecore Commerce 9 initial release:
- http://tothecore.sk/2018/02/07/installing-sitecore-experience-commerce-sxc-9-0-initial-release. Should be very similar.