Whether you buy a new ringtone, select one, or create your own, there are several ways to customize your iPhone ringtone.
Steps
Method 1 of 2: Add new ringtones from iTunes

Step 1. Connect your iPhone to your computer and open iTunes
Select your iPhone in iTunes by clicking the button at the top right.

Step #2. Click on Ringtones and make sure there is a checkmark in the Sync ringtones box

Step 3. Click All Ringtones or Selected Ringtones
If you choose the second, manually select the ringtones you want to sync. Click Apply to start syncing.
Method 2 of 2: Set a new ringtone

Step 1. Open your iPhone's Settings

Step #2. Press Sounds

Step #3. Now tap on Ringtones under the Sounds and Vibration Patterns heading

Step 4. Scroll through the list of available ringtones and press the title of the tone to listen to it or set it as a ringtone
Tips
- If you have ringtones on your computer, you can put them on your iPhone by putting them in iTunes and then synchronizing them with your iPhone. They will then appear among your ringtones.
- GagageBand lets you create your own ringtones on a Mac or iOS device.
- Ringtones you've added or purchased in iTunes will be at the top of your ringtones list.