I believe you can set the default user like this:

<blockquote>
<b>fossil user default</b> <i>user-id</i>
</blockquote>

The default user set as above overrides any USER environment variable setting.
You an also set the user for individual commands using the -U or --user command
line switch.  For example:

<blockquote>
<b>fossil commit --user </b>
</blockquote>

The -U or --user switch overrides the user set by the "user default" command
and the "user default" command overrides the USER environment variable.