# backup
cp /etc/opkg/distfeeds.conf /etc/opkg/distfeeds.conf.backup

# tuna
sed -i 's_downloads.openwrt.org_mirrors.tuna.tsinghua.edu.cn/openwrt_' /etc/opkg/distfeeds.conf

# or cqu
sed -i 's/downloads.openwrt.org/mirrors.cqu.edu.cn\/openwrt/g' /etc/opkg/distfeeds.conf

# full upgrade
opkg list-upgradable | cut -f 1 -d ' ' | xargs -r opkg upgrade