Clearing program

please anyone can help me?
I am using arduino uno/ethernet shield too, i am using relay with it, i upload the code and now the program can not be overrided, it always run the first program that i uploaded,when i remove the shield the arduino works just fine please how i can remove the previous program from shield?

Shields usually don't contain programs. Which shield (link please).

Remove shield, upload, then place shield back on.

I think ethernet shields interfere with the upload. OR there is something on digital pins 0 and 1.