Thank you very much for the help. I found that I was not using the find command properly ( I am an absolute beginner after all!). When I finally managed:
gentoo / # find -name webmin
I got a list that included: ./usr/libexec/webmin
Now knowing the directory, I tried:
# /usr/libexec/webmin/changepass.pl /etc/webmin admin newpassword
that I found here: http://www.enterprisenetworkingplane...0954_3550291_2
An error message told me the only user was "root" so using the same command I changed the password.