The M6 has a microSD memory card slot. If your personal computer has a slot for that card type (Most notebooks are so-equipped), you should be able to save music files
The M6 is equipped with Bluetooth version 4.1 (LE), which lets you connect to your computer wirelessly. Just pair phone and computer together and drag and drop any music file from your computer to your Meizu M6.
If the above-mentioned techniques fail or if you prefer not to buy new accessories, you can always use one of the numerous free '3G Uploader' services available online. A search on Katapulco should retrieve plenty of them. 3G uploaders are simple: Transfer your own ringtones on their web site and they will provide a URL. Navigating to that URL with your Meizu M6's browser will download the free ringtone.
More on free ringtones
Here are links to some of the 100% free ringtones (Meizu M6-compatible) offered here at MartinsRingtones.com:
Free 'Unclassified' Ringtones Free Tropical Ringtones Free Ringtones-a-gogo Free Techno Ringtones Listen To All Our Free Polyphonic Ringtones