Although it isn't possible to uninstall a preloaded app from your device, you can disable it to prevent it from loading on your device.
- First, go to your Android Settings.
- Under Security, tap Device administrators and uncheck the box next to F-Secure Mobile Security.
- Then, in the main Android Settings find Apps.
- Tap F-Secure Mobile Security and choose "Force Stop"
- Tap Storage then "Clear Data"
- Lastly, click the back button and tap "Disable"
Some more background:
For some users, F-Secure Mobile Security is preloaded on their device and it is therefore considered a system app, the same as Facebook, Flickr, Google Maps, and many others. System apps cannot be uninstalled from a non-rooted device. You can validate this by going into the "Manage Applications" setting and choosing "F-Secure Mobile Security" or "Google Maps." You will see that neither provides an uninstall option (other than uninstall updates).
If you have gone to the Google Play Store and updated F-Secure Mobile Security from there you can remove those updates from your phone, but not the base app itself. Again, this works the same way for all system apps.