The A5 features a microSD memory card slot. If your personal computer has a slot for that type of card (Most notebooks are so-equipped), you should be able to save music files
The A5 is a smartphone. Most smart phones let you save any mp3 song available online by touching and holding your finger on it, then selecting 'save.' Once the music file is stored on your library, tap-and-hold it and select 'assign' in the contextual menu.
The A5 is equipped with Bluetooth version 4.2, 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 Lenovo A5.
If the above-mentioned tricks 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 work as follows: Transfer your own ringtones on their web site and they will provide a URL. Navigating to that URL with your Lenovo A5's browser will download the free ringtone.
Here are links to some of the free ringtones (Lenovo A5-compatible) offered here at MartinsRingtones.com:
Free Horror MIDI Ringtones Free Tropical Ringtones Free Ringtones-a-gogo Free Techno Ringtones Listen To All Our Free Polyphonic Ringtones