You need to write the changed string back out to a file, you could do this, and avoid loading all those libraries you don't need too. Loading "arcpy" takes forever.
import os
IN_workspace = r"H:\PROGRAMMES\10_OTHER_PROJECTS\26_04072022_KEV"
OUT_workspace = r"H:\PROGRAMMES\10_OTHER_PROJECTS\26_04072022_KEV_NEW"
Lyrx = "AE000S570_005M5001P01M041.lyrx"
with open(os.path.join(IN_workspace, Lyrx), "r") as fp:
lyrx = fp.read()
fixed = lyrx.replace('570,\n"max"', '560,\n"max"')
if lyrx == fixed:
print("WARNING, NOTHING CHANGED!")
with open(os.path.join(OUT_workspace, Lyrx), "w") as fp:
fp.write(fixed)
I used "os.path.join" to build up the file paths just because it is tidier, sometimes that avoids problems with missed slashes and stuff like that. It's OS independent too.
I avoid relying on the arcgis "workspace" because about 1/2 the Esri tools just ignore it.