flip words in a list

Actually, the above method is not making a change in the actual list. Try this:-

[word[::-1] for word in mylist]

Output:-

['licnep', 'god', 'omem', 'olleH']

You should use another list to save your changes because you should not modify the same list which you are looping over.
So you can fix your code like this:

mylist  = ['pencil', 'dog', 'memo', 'Hello']
new_list = []

for word in mylist:
    word = word[::-1]
    new_list.append(word)

print(new_list)

Another way to reach the same goal is to use list comprehension.
Also in this case you are basically created a new list like this:

mylist  = ['pencil', 'dog', 'memo', 'Hello']
newlist = [word[::-1] for word in mylist]

print(new_list)

The problem with your code is that you are changing the word but not storing it in current list. You can use following script, if you want change in current list

mylist  = ['pencil', 'dog', 'memo', 'Hello']

for word in range(len(mylist)):
    mylist[word] = mylist[word][::-1]
print(mylist)

Output

['licnep', 'god', 'omem', 'olleH']

You should create a new list and append the flipped words in the new list.

Check the following code below:

mylist  = ['pencil', 'dog', 'memo', 'Hello']

newlist = []
for word in mylist:
    word = word[::-1]
    newlist.append(word)

print(newlist)

Your code did not work because - For each iteration of the for loop, the variable word is assigned with just a copy of the value of an item in mylist, so changes made to word won't be reflected in mylist. Here word is just a local variable and it does not reference an item in mylist.