Python - How to change values in a list of lists?

You could use enumerate():

for index, sublist in enumerate(execlist):
   if sublist[0] == mynumber:
       execlist[index][1] = myctype
       execlist[index][2] = myx
       execlist[index][3] = myy
       execlist[index][4] = mydelay
       # break

You can remove the # if execlist only contains at most one sublist whose first item can equal mynumber; otherwise, you'll cycle uselessly through the entire rest of the list.

And if the itemnumbers are in fact unique, you might be better off with a dictionary or at least an OrderedDict, depending on what else you intend to do with your data.

The problem is that you are creating a copy of the list and then modifying the copy. What you want to do is modify the original list. Try this instead:

for i in range(len(execlist)):
    if execlist[i][0] == mynumber:
         execlist[i][1] = myctype
         execlist[i][2] = myx
         execlist[i][3] = myy
         execlist[i][4] = mydelay