With the iPhone XS, iPhone XR, or the new iPad Pro, you can use Apple’s own Photos app to adjust the amount of background blur in a Portrait Mode photo even after you take it. Unfortunately, you don't have that capability if you have an earlier iPhone or iPad: You can enable or disable Portrait Mode, but not change the blur.
A few third-party photo apps let you do the trick, but none are as popular or useful as Google Photos, one of the best photo archiving apps for iOS.
Today, Google announced on Twitter that Google Photos now lets you edit any Portrait Mode photo on any device. You can tap to choose a new focal point and drag sliders to adjust the amount of blur or foreground blur.