To sign out of fomo in the app, open the Portfolio tab (its icon is your avatar), tap the gear icon to open Settings, then scroll to the bottom and tap Logout.
On the web, hover your profile picture at the top right and choose Log out at the bottom of the menu.
Signing out removes your active session on that device. Your wallet, funds, and positions are not affected. Everything is tied to your Apple ID or Google Account and will be there when you sign back in.
If you want to sign out from a device you no longer have access to, change the password on the Apple ID or Google Account you used to sign up. After changing your password, sign back in on your current device with the updated credentials.
fomo does not have a "sign out from all devices" button. Updating your Apple or Google password is the way to secure your account if you believe someone else has access.