two places
compmgmt.msc > connect to the remote computer you want
users and groups
users
administrator (right click > rename)
other one is gpedit.msc
computer configuration
windows settings
security settings
local policies
security options
4th option i think for renaming the administrator account
i think the command you're speaking of may be net user or something, net use is for mapping network drives, i'll see if i can find it
EDIT: net user /? in cmd prompt gives u the options, but it's used to change the account password, not the name or membership