problem regarding matrix analysis

i want to write a code that will work as :

-> i have a matrix (normally 512*512) with any random values.

-> a threshold value.

-> i want to start a loop so it will check all the values . if a value is greater than the threshold value it will leave it without any change.

-> but if a value of a particular matrix entry is less than the threshold value than it will check the neighbouring entries of that entry and add it in the entry which has the greatest value and place a null value in the original entry.

please help me writing this code i dont have any idea how to find the greatest neighbouring entry

:S