Backup binaries:

cp /usr/sbin/tailscale backup_tailscale
cp /usr/sbin/tailscaled backup_tailscaled

Update (https://pkgs.tailscale.com/stable/#static):

service tailscale stop

wget https://pkgs.tailscale.com/stable/tailscale_1.50.1_mips.tgz

service tailscale stop
tar zxvf tailscale_1.50.1_mips.tgz

cp /root/tailscale_1.50.1_mips/tailscale* /usr/sbin/
service tailscale start
tailscale version