Log into a different Tailscale account or tailnet and switch nodes with tailscale login / tailscale up

Documented steps

1. To add or switch the current device to a different account/tailnet without fully resetting, use `tailscale login` (this is the dedicated login command; Tailscale supports Fast User Switching via the --login-server / account switchers on desktop UIs).
2. Run `tailscale login` on an already-installed node to re-authenticate or switch identity; it prints a URL to sign in, or accept an auth key with `tailscale login --authkey=$KEY`.
3. After login the node joins the credentials' tailnet; confirm with `tailscale status` and `tailscale whois` to see the current user/tailnet.
4. To depart the tailnet entirely, run `tailscale down` — it disconnects the node from the tailnet without uninstalling; run `tailscale up` again to reconnect.
5. To switch between multiple tailnets and know which is active under Fast User Switching, check `tailscale status` (shows the current tailnet) or use the desktop account switcher; note that `tailscale set` changes apply only to the currently connected tailnet.
6. To fully reset a node to defaults before rejoining a different network (including clearing exit-node/routes), use `tailscale up --reset` — but be aware flags not passed in that command reset to defaults.