One approach:
Before = [ 1 9 9 9 3
8 9 9 9 4
8 9 9 9 4
8 9 9 9 4
7 9 9 9 5]
After = Before;
After(1,2:end-1) = ones(size(Before(1,2:end-1)))*2
producing:
After =
1 2 2 2 3
8 9 9 9 4
8 9 9 9 4
8 9 9 9 4
7 9 9 9 5