If you’re on the default gts tag you’ll be moved to F39 when F40 comes out. It’s the part in the rpm-ostree status after the colon and looks like this: docker://ghcr.io/ublue-os/bluefin-dx:gts
If you want to be on F39 explicitly you can rebase with this:
rpm-ostree rebase ostree-image-signed:docker://ghcr.io/ublue-os/bluefin-dx:39
Or if you always want to be on the latest release you can use latest, which is currently 39:
rpm-ostree rebase ostree-image-signed:docker://ghcr.io/ublue-os/bluefin-dx:latest
That will always keep you on the latest version when it’s released, which also means that you’ll move to F40 the day it comes out so heads up on that!