Correcting an upload error

I'm very new to using the Arduino and the IDE. I purchased two Arduino's. I uploaded a program to one and made a mistake. The program uploaded but one instruction was wrong. I thought I could just upload again to correct the mistake, but it will not change. So I tried the other Arduino with the corrected program. This program works fine. SO, my question is how to make a change. Must I clear the Arduino memory, or ????? Thanks Mike

No.

OK, so I should be able to over write the memory? Mike

Every time you upload a new sketch, the previous sketch is erased.

mike_zahorik:
I thought I could just upload again to correct the mistake, but it will not change.

Please explain what you mean by "it will not change".

Thanks, I thought so. I have the proper program in the Arduino now. Not sure what the trouble was, but here is what I did.
In preferences I checked 'Verify code after upload'
And 'show verbose output during upload'

This is some of the red comments
avrdude: 8202 bytes of flash written
and later
avrdude: 8202 bytes of flash verified

So to me that means the program as I had it in IDE was proper transferred. Then after this the program worked fine. I can't explain it. Maybe I had a bad connection somewhere. Mike