Your F9 features a microSD memory card slot. If your personal computer is compatible with that type of card (Most laptops are so-equipped), you should be able to save a music file
The F9 is a smartphone. Most smart phones let you save any song found online by tapping and holding your finger on it, then selecting 'save.' Once the music file is saved to your media library, tap-and-hold it and select 'assign' from the drop-down menu.
The Oppo F9 is an Android phone and there are plenty of free ringtone apps such as MP3 Cutter, RingDroid (open source) and more.
The Oppo F9 is Bluetooth-enabled. Bluetooth is a wireless system which lets you connect your cell phone to computers, headsets, car kits, etc. If your computer is also Bluetooth-enabled, simply establish a connection between both devices and transfer your music files from your computer to your Oppo F9.
Here are some of the free ringtones (Oppo F9-compatible) offered here at MartinsRingtones.com:
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