Replace an "Except" element of a matrix under a Table?

Questions
1

Dear Friends,

In a “For” operation, I have a table defined by indices i & j. Any (i, j)th element of this table consist of the elements of matrix f[m] (6X1 matrix), where m is the element (Integer) on which “For” loop is running.
During the “For” loop I want to replace the terms f[m][[k] by f[m-1][[k]], where k is a number other than the indices i & j for the (i, j)th element.
When I tried to use

`/.f[m][[X-]] -> f[m-1][[X]]`,

I am getting the following error message –

Part::pkspec1: The expression x$_ cannot be used as a part specification.

Could there be an alternative way to achieve this ??

thanks for the help in advance !

2

It would be helpful to see your code to get an idea about your problem.