This page opens already anchored to A = 432 Hz. There is nothing to change first: every note, every reading and every reference tone is re-anchored the moment it loads, and the anchor is shown next to the instrument name so you can see it is set.
Concert pitch is A = 440 Hz, and 432 sits about 32 cents below it, roughly a third of a semitone. Equal temperament scales linearly with that anchor, so moving it moves every string together rather than only the A.
The setting persists, so it will still be at 432 next time. Step it anywhere between 430 and 450 under settings, or tap the 440 preset to go back to standard.