What to do when you discover computational errors and mistakes in your accepted paper at the proof stage? My paper is accepted with minor revisions and now is in proof stage. Unfortunately, I have found computational errors in my calculations which have led to some wrong results.  

What should I do?
Sometimes mistakes happen. Yes, it's a pain to have to deal with this mistake now after the paper was accepted for publication, but it's a good thing that you caught your mistake before the paper was published. 

Note: If you have coauthors, before doing anything, contact your coauthors and explain to them the changes you need to make. Do this before doing anything else.


  What should I do? Should I send the errata to editor or copy editor?


As you mentioned that you are dealing with an IEEE journal, my response here is specific to the IEEE:

Talk to the senior editor that you are working with currently. Tell them specifically the changes you want to make and that "the logic of the concluding remarks is still correct and the discrepancies only impact the affected numerical results and the specific conclusions drawn from them," and that "the other numerical results are correct." 

The senior editor will take it from there. Based on my past experience, you do not need to contact the editor-in-chief about this directly.


  Will it affect my paper's value? 


No, but I don't really follow why you think correcting a mistake will affect your paper's value. Correcting a mistake prior to publication is a good thing.


  Will it need to go through another review cycle?


This will depend on the steps taken by the senior editor.