how to change to sub-directories URLs for MU Network

I don’t recall being given a choice between sub-domain and sub-directory URLs for my Network sites during the install/set-up. Right now it is set up as sub-domains but I would prefer sub-directories so the site addresses are: https://mydomain.com/samplesite instead of https://samplesite.mydomain.com

Can someone walk me through what I need to do to reconfigure my install? Thank you!!