I miss İstanbul a lot. It means 'İstanbul'u çok özlüyorum' .......(present tense)
if you want to say 'I missed İstanbul a lot'. You should say 'İstanbul'u çok özledim'....(past tense)
So about the suffix 'u' ,we use it to indicate objects in the sentence.Without using it,the sentence will not be understanable that you point out by verb.
Here are some other examples:
Evi özledim.......I missed home
Babamı özledim....I missed my father.
Okulu özledim.....I missed school.