Q: I was reading Bukhari, and I find Umar radhiallahu anhu changed so many Sharia laws such as muta, poetry in Musjid, and he even changed taraweeh. Explain why we follow such a man?
A: Do you know of the virtues mentioned by Rasoolullah (sallallahu alayhi wasallam) about Hazrat Umar?
The hadith states: "The Haqq flows from the tongue of Umar" and "If there was to be a Prophet after me,it would have been Umar". These sayings of Our Rasool (alayhis salaam) prove that Umar had vast knowledge and vision. Hazrat Umar was responsible for setting up one of the most effective and comprehensive systems of government, based on Quran and Sunnah; a system that today even the kuffar are learning from.
Umar (radhiyallahu anhu) did not "change" any Shariah laws. You are highly mistaken when you say "he changed so many Shariah Laws". As for Muta, it was only allowed for a short while then banned, right in the time of Rasoolullah (sallallahu alayhi wasallam). He prohibited poetry that was not in conformity to Shariah, so again he did not do anything different to what The Messenger of Allah had done. There are numerous hadith on maintaining the respect of the Musjid.
Regarding Taraweeh, here too, he changed nothing. Taraweeh was started by Allah's Messenger. All Umar did was to fix a number of Rakaats behind one Imam. This was accepted by all the Sahaaba so it was not just the vision of Umar alone. Even Hazrat Ali (radhiyallahu anhu) praised him and made dua for him when he saw people offering congregational Taraweeh in the Masaajid during the nights of Ramadaan.
Another hadith in Tirmizhi states: "Cling fast to my Sunnat and the sunnat of my rightly guided Caliphs after me." In this hadith it is clearly stated that Umar's sunnat has to be followed and is like the Sunnat of Allah's Messenger himself.
Why are you singling out Umar for this unwarranted criticism, when it was Hazrat Abubakr who compiled the Quran for the first time in book form, between two covers, and then Hazrat Uthman did the same. It was Hazrat Uthman who introduced the first athan of Jumu'ah outside the Musjid. These issues were not done by Allah's Messenger. So Umar is not alone in the introduction of important issues that affect the Ummat. If you start rejecting the practices and fatwas of the Sahaaba, you won't have any Deen left.
My advice to you is to stop reading Bukhari for you clearly don't understand the essence and reality of the ahadith in Bukhari. Rather go and sit in the company of a qualified Aalim and learn your Deen there. That will be safer and far easier