$ sudo apt-get install flatpak

$ flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
$ flatpak remote-modify flathub --url=https://mirror.sjtu.edu.cn/flathub

# 或者
$ flatpak remote-add --if-not-exists sjtu  https://mirror.sjtu.edu.cn/flathub/flathub.flatpakrepo