This script starts with the brute clearing header "clc; clear all; close all;". This is a bad idea, because "clear all" deletes all breakpoints. Impeding the debugging is a very bad idea.
The algorithm is slow: The builtin function MAX is much faster than SORT and selecting the last element.
Expressions line "[d(i-1,j-1) d(i-1,j) d(i-1,j+1) d(i,j-1) d(i,j) d(i,j+1) d(i+1,j-1) d(i+1,j) d(i+1,j+1)]" can be accelerated: "d(i-1:i+1,j-1:j+1)".
The construction of "m=(1/9)*(ones(3,3));" can be omitted, because "m" is not used anywhere.
A function with inputs and outputs would be more useful than this script, which obtains the image by UIGETFILE. Without any comments and documentation the usability of this code is very low.