Are you talking about Librem 5? If so, no, you don’t have to reflash. Reflashing is used when you want to wipe absolutely everything out and start again. If you just want to “push a button” then read this post: PureOS Crimson is released!!! and specifically
Existing Byzantium devices will automatically receive a PureOS Upgrade tool soon when installing system updates. New devices will begin shipping with PureOS Crimson pre-installed.
Byzantium users wishing to use this tool today can run:
sudo apt install pureos-upgrade
So, based on that, “push a button” would mean that you can’t quite do it yet, but “soon”.