To edit a user from the participant administrator, there are two ways. The first is to right click on the user and select 'Edit'. The second is to click on the Info icon next to the user's name. From the Info page, go to Actions->Edit.
However, if the user is coming from a read only LDAP, then the edit option will not be available in either of these places. Generally, Windchill will only have read access to corporate directories, like Active Directory, so Windchill cannot edit these users. In that case, changes to the user must be done on the directory server side.