The Official Method: How to Change MP3 to MR4 with iTunes?
Step 1 Launch iTunes on your computer and then click on Files > Add File to Library option.
Step 2 Click your MP3 files and select Get Info > Options, then it will pop up another window as below, so you need to select the starting and ending time of your ringtone.
Notes: Make sure that it is not more than 40 seconds, and you can apply your settings in order to clip the file.
Step 3 After clipping the audio file, you need to perform mp3 to m4r conversion. Just go to the "File" on the top left menu and select the option of “Create AAC version” from Convert tab. This will convert your file to m4r format.
Step 4 When it is done, go to the media location and change the extension of .m4a to .m4r.
Step 5 Sync the tones in the iTunes with your iPhone and your custom ringtone will be applied in the iPhone.
Best iTunes Alternative: How to convert MP3 to M4R for your iPhone Ringtone?
Without iTunes, how can you convert MP3 to iPhone ringtone without hassle? Luckily, here you can try to use this awesome mp3 to m4r converter - iOS Transfer. It can help you to customize your ringtones and convert mp3 to m4r easily, and it will perform mp3 to m4r conversion automatically while saving a ringtone on your device. What's more, it also provides plenty of other features which let users personalize their ringtones with ease.