On my end, https://fly.io/install.sh is giving a 502 error. I have tested this across multiple connections and ISPs.
Because of this, updating flyctl is not possible.
$ flyctl version update
Update available 0.0.451 -> v0.0.462.
Run "flyctl version update" to upgrade.
/bin/bash -c
Running automatic update [curl -L "https://fly.io/install.sh" | sh]
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0