Let's asume that by some reason you have been sent to the past, be it Rome, Greece, Egypt, a civilization from an alternative Earth, or any other civilization that practiced slavery, aka, practically any human civilization that developed agriculture (Humanity, fuck yeah!).
Now, for some reason you have the power to influence the course of the nation. Maybe you were made into king/queen/emperor/emperatriz/sultan/pharaoh/whatever in your travel to the past, or you are have become an advisor thanks to your inexplicable good education and view of the world (as well as your inexplicable ability to understand the local language).
Now, the thing is, live sucks for everybody except you and some rich lucky few.
To put it symply, you can expect 80% or more of the population to be slaves.
Most of them are farmers, (peasants and serfs). Uneducated, they are bound to work the land of their lord in exchange of protection (Unless it is like a dictatorship).
Then there are those that are a luxury, servants working directly under a high class citizen.
Lastly, there are those heavily discriminated, that can't even work the land, or criminals, who are sent to work on the mines until they die.
Your objectives would be giving better rights to the masses, improving living conditions, giving the peasantry a voice, abolishimg slavery (without harming the economy and army, no point if you become an easy target to be conquered), making a society based on merit (and lessening racism), gender equality (by it originally a matriarchal or patriarchal culture), making education a right, not privilege, probably making a centralized government, and preferably keep the crown in power (or another method of fight off corruption.)
Also, make an army that doesn't kill, plunder, and rape every city they conquer would be nice (establish war crimes).
You have access to all the information about farming techniques, engines, industrialization, medicine, anything you can find in the internet.
You kingdom is feudal, so you would have to be careful if taking the power out of the nobility, and guilds are a thing.
Now, how would it be possible to enlighten this civilization to more fair and liberal society?
And once this changes have been set, how would you deal with a word that keep such practices?