Redox reaction

pH = −log [H+] , where [H+] denotes the molar hydrogen ion concentration. Notice that we are required to take the common (base 10) logarithm of the hydrogen ion concentration in order to calculate pH.

In the question I post in file How can I find ph in that question

By taking reaction at anode and cathode we will get O2 at anode and Cu at cathode so H+ left in solution so ph decrese

simply while eletrolysis the water gives protons so proton concentration increases ph decrease