As shown in the screenshots below, the OpenTunnel Designer Working Units are set by the chosen Seed files, just like any other .DGN file, when creating a new project or file.
Therefore, to modify to a different set of units the best option is to make it when initializing the file, at the Configuration Tab:
If the user is already inside a specific .DGN file, and the units are not right, then it can be edited to the right ones. The user has to go to the File Tab, and proceed with the modifications as highlighted in the images below:
More technical information can be found at the links below, from our online documentation:
Important MicroStation Concepts (bentley.com)
File tab and Backstage View (bentley.com)
Information about the Dimensions Styles inside MicroStation-based products (PowerPlatform) can be found at the link below: