If your ringtones are in another format, you may need to convert them to .m4r using a third-party conversion tool.
Using a Windows PC:
1. Connect your iPhone 13 Pro Max to your PC using a USB cable.
2. On your PC, open iTunes (or download it from the Microsoft Store if you don't have it installed).
3. Once iTunes is open, click on the device icon located near the top left corner of the iTunes window.
4. In the left sidebar, under the "Settings" section, click on "Tones."
5. Now, simply drag and drop your .m4r custom ringtone files into the iTunes window under the "Tones" section.
6. Once you've added your custom ringtones, click on the "Sync" button in the bottom right corner to transfer them to your iPhone.
7. Wait for the sync to complete, and then disconnect your iPhone from the PC.
Using a Mac:
1. Connect your iPhone 13 Pro Max to your Mac using a USB cable.
2. Open Finder on your Mac.
3. In the Finder window, click on your iPhone under the "Locations" section in the sidebar.
4. In the top toolbar, click on "File" and select "Show Sidebar" if the sidebar is not already visible.
5. Under the "Settings" section in the sidebar, click on "Tones."
6. Now, simply drag and drop your .m4r custom ringtone files into the Finder window under the "Tones" section.
7. Once you've added your custom ringtones, wait for the sync to complete.
8. After the sync is finished, you can disconnect your iPhone from the Mac.
Once you have followed these steps, your custom ringtones should be available on your iPhone 13 Pro Max. To set a custom ringtone for a specific contact or for your general incoming calls, you can go to "Settings" > "Sounds & Haptics" > "Ringtone" on your iPhone.