Question

What were the major changes introduced by Stalin in Russia​

Answer 1

If we mention both bad and good changes, then we can say that his introduction of 5-year economic plan wasn’t bad, his police (NKVD) became one of the most efficient in the world at that time (at the same time, the cruelest), his repressions led to deaths of many people (both peasants and high-ranked leaders), lots of people were also sent to GULAG during his years. Joseph Vissarionovich Stalin was also a great commander, still killed many of people.

Answer 2

I can think of two examples:

Initiation of a planned economic environment. Certainly, initial attempts (such as the GOELRO plan - Wikipedia) were undertaken before he came to power, but his determination and perseverance allowed the Soviet Union to defeat the Nazis and reach the moon.
(To provide some background information, Leonid Kantorovich - Wikipedia, who was the de-facto inventor of the Soviet planned economy, was awarded the Nobel Prize for economics in 1975.

a prize given by Stalin (aka USSR State Prize - Wikipedia.) Primarily, Stalin was able to do so because he got royalties from the publication of his books. As an alternative to sitting on a large sum of money, he opted to give his money to scientific research, artistic creation, and engineering.