Unlike other fields, goal of studying upanishads is never to be a ‘technical-expert’ by knowing all Upanishads, instead goal is to “find own path by resolving all questions”, so that the mind becomes calm for making spiritual progress.

Best advice on how to go about Studying upanishads is given in Muktika Upanishad. For the people who don’t know, there are in-all 108 upanishads and Muktika Upanishad acts like a Table of Content, listing all the upanishads.

As a part of listing all the Upanishads, Muktika Upanishad captures discussion between Hanuman and Shri Ram, where Hanuman taking Ram’s advice on, ‘how one can go about spiritual progress and achieve moksha’. 

Shri Ram advices Hanuman:
इयं कैवल्यमुक्तिस्तु केनोपायेन सिद्ध्यति । मांडूक्यमेकमेवालं मुमुक्षूणां विमुक्तये ॥
तथाप्यसिद्धं चेज्ज्ञानं दशोपनिषदं पठ । ज्ञानं लब्ध्वा चिरादेव मामकं धाम यास्यसि ॥
तथापि दृढता नो चेद्विज्ञानस्याञ्जनासुत । द्वात्रिंशाख्योपनिषदं समभ्यस्य निवर्तय ॥
विदेहमुक्ताविच्छा चेदोष्टरशतं पठ । तासं क्रमं सशांतिं च श्रुणु वक्ष्यामि तत्त्वतः ॥

If one can read and understand Mandukya Upanishad alone, that is enough for attaining the moksha, nothing else needs to be studied. (Btw, Mandukya Upanishad is just 12-shloka power packed enough to help seeker achieve moksha.)

But, If questions are not resolved, then read the ten primary unpanishads, which include Isha, Kena, Katha, Prashna, Munduka, Mandukya, Taittariya, Aitreya, Chandogya and Brihadaranyaka.

If the questions are still not resolved, then read 32 and stop. One need to read all 108 only if he/she wants to achieve Videhamukti.

In summary, each of us are different, with different levels of curiosity, aptitude and inclination, and that is the reason all these various upanishads exists. We must follow own curiosity, read needed Upanishads and find the path that meets our needs.