Resolution and enlargement

If you are looking to intelligently upscale your images, you must use Lightroom for Mac or PC.  See more info here: https://www.adobe.com/products/photoshop-lightroom/super-resolution.html 

If you have only Lightroom Mobile, you can stretch your images to a larger pixel dimension during export. This stretches pixels, and is not a true upscale of the image.  If you still want to give it a try: 

Open an image, tap Share > Export As.. and under Dimension, switch to Custom, and choose the pixel amount you want for the longer side of your image.  Doing this myself just now, I was able to convert a 12 MP image (3024x4032 px) to 4500x6000