The output result of cnn is like(a,b,c).how can i use "a" to recaculate the "c" ,and output(a,b,new_c)

the output result of cnn is like(a,b,c).how can i use “a” to recaculate the “c” ,and output(a,b,new_c). and i want to achieve in inside of the network ,how can i do .