3500c

Using your phone's cable, connect your phone to the USB port of your computer.
Your Nokia 3500c will ask which the mode in which it should connect to the computer.
Choose Data Storage
Your phone will appear as a new removable drive on your computer.
Copy the mp3 from its location and paste it inside any folder of your phone.
Disconnect your phone by choosing "Safely remove hardware" from the bottom right corner of your computer's taskbar (Windows).
On your phone, open the Main Menu .
Then select Settings
Next, select Tones
Scroll down to the field which says "Ringing tone" and select "Change"
Select "Open Gallery"
Locate the mp3 file you just transferred and select "Select"
Scroll down till you reach the "Save" button and choose "Save"