There are two ways to setup a multisite network, using subdomains and using subdirectories. Your current setup is for subdirectories and to change the URLs the way you want them you‘ll need to change the setup to use subdomains. This is a great article explaining the difference between those two setups and how to change from subdirectories to subdomains: https://premium.wpmudev.org/blog/change-multisite-sub-directories-sub-domains/
Beware that to setup WordPress multisite to use subdomains you’ll need to use wildcard subdomains. Make sure your hosting provider can set that up for you.