@tingping - Thank you also for your reply.
I am sorry but I am an end user of flatpak and do not now what it means to test runtimes.
I tried to learn more about what you mean by eg searching for:
flathub install older runtime
and found articles like:
in which the OP does not understand things either
and
Downgrading | Flathub Documentation which seems to be oriented to developers eg using terms like “Then you deploy the commit:” which I have no idea of what this would mean.