Cannot be changed. The default is 240ppi.
Why is PPI important to you?
Changing the PPI does not change the 'Pixel' dimensions of an image.
Changing the PPI does not change the 'Quality' or 'Resolution' of an image.
Regards. My System: Lr-Classic 12.3 Photoshop 24.3, ACR 15.3, Lightroom 6.3, Lr-iOS 8.0.8, Bridge 13.0.3, Windows-11.