How do you change a villager profession in Minecraft?

Remove its job site block and place a different one nearby. The villager claims the new block and changes profession. This only works while you have never traded with it; after the first trade the profession and trades lock forever.

How do villager restocks work?

Villagers restock their trades up to twice per day while working at their job site block. In Bedrock Edition the villager also needs to be linked to a bed to restock.

How do you get discounts on villager trades?

Cure a zombie villager for a large permanent discount from that villager, gain the Hero of the Village effect from beating a raid for a 30% discount, and keep trading to build positive reputation. Attacking villagers raises prices.

Minecraft Villager Trades

All 13 professions with complete trade tables, verified against the Minecraft Wiki. Java and Bedrock.

Quick answer: Give an unemployed villager a job by placing a workstation block next to it (a lectern makes a librarian, a composter a farmer...). Trade to level it up from Novice to Master and unlock better trades. Cure zombie villagers for permanent discounts, and never hit villagers you trade with.

All professions

Librarian

Workstation: Lectern

Farmer

Workstation: Composter

Armorer

Workstation: Blast Furnace

Toolsmith

Workstation: Smithing Table

Weaponsmith

Workstation: Grindstone

Cleric

Workstation: Brewing Stand

Cartographer

Workstation: Cartography Table

Fletcher

Workstation: Fletching Table

Butcher

Workstation: Smoker

Fisherman

Workstation: Barrel

Leatherworker

Workstation: Cauldron

Mason

Workstation: Stonecutter

Shepherd

Workstation: Loom

How trading works

Levels

Villagers start at Novice and level up as you trade: Apprentice (10 XP), Journeyman (70), Expert (150) and Master (250 villager XP). Their badge changes from stone to iron, gold, emerald and diamond. Each level unlocks new trades: up to 2 new trades in Java, 1-3 in Bedrock. When a level offers a pool of possible trades, the villager picks randomly - which is why two librarians offer different books.

Locking and changing trades

Before you trade with a villager, you can break and replace its workstation to reroll its trades (the classic way to fish for a Mending librarian). The moment you complete one trade, the profession and every trade lock forever. Novice villagers that never traded can also lose their profession if the workstation is removed.

Restocking

Each trade has a limited stock (usually 12-16 uses). Villagers restock up to twice per day while working at their workstation. In Bedrock Edition they must also be linked to a bed. A sold-out trade shows a red X (Java) or a red slot (Bedrock) until the next restock.

Prices: demand, reputation and discounts

Demand: buying out a trade repeatedly raises its price; leaving it alone lets the price fall back.
Reputation: trading builds positive reputation (small discounts); hitting or killing villagers does the opposite, and golems turn hostile if your village reputation drops far enough (Java).
Curing: curing a zombie villager (Splash Potion of Weakness + golden apple) gives that villager a large permanent discount, stacked further in Bedrock when other villagers gossip about it.
Hero of the Village: beating a raid cuts all prices by 30% (plus 6.25% per extra level) while the effect lasts.

Nitwits and unemployed villagers

Green-coated nitwits can never take a job. Unemployed villagers claim any free workstation in range - trap one, place the block you want, and you have the profession you need. Baby villagers cannot work; in Bedrock they grow into nitwits 10% of the time.