C:\Users\thomas\Desktop>gem sources --remove https://rubygems.org/
source https://rubygems.org/ not present in cache

C:\Users\thomas\Desktop>gem sources -a https://gems.ruby-china.com/
source https://gems.ruby-china.com/ already present in the cache

C:\Users\thomas\Desktop>gem sources -l
*** CURRENT SOURCES ***

https://gems.ruby-china.com/