Valheim world modifiers did not change

World rules are written into the world at the next start. If nothing changed, the server was probably not restarted, an earlier rule is still stored in the world, or the rule went to another world. Open Worlds, World rules: the table At the next start shows what the world has stored now and what it will have after the start.

Check these in order

1. Did you restart? The rules apply at the next start. On the World rules tab, Save and restart does both. Save, restart later waits for your next restart.
2. Did it save? The card At the next start shows a Saved badge, or Not saved if you still have changes.
3. What does the world have stored? The column Stored now shows the rules the world has after its last start, and After the start shows what it will have. Highlighted rules change when the server starts. If the page says The world keeps the rules it has., nothing you chose changes the world.
4. Is it the right world? The card names the world under World. Rules apply to the world the server runs, the one marked Active on the Worlds tab. See How to switch between Valheim worlds on your server.
5. Did the server start? The Console shows lines such as Setting world modifier preset: hard while it applies them, then ZNet.LoadWorld: and Online.

Undo a rule

Choosing Keep, or turning a switch off, leaves the old value in the world. The page says: Keep never undoes a rule the world already has. To go back to the game as designed, choose Normal.

1. Pick the Normal preset.
2. Set the five Fine-tune rules to Keep and turn the switches off.
3. Click Save and restart. Afterwards you can pick Keep what the world has again.

Full steps and the list of rules: How to set Valheim world modifiers and difficulty.

A preset replaced your rules

A preset is applied first and replaces the rules set before it. The Fine-tune rules are applied after it. Set both in the same save if you want a preset with one change on top.

A world you switched to or uploaded

When you switch with Play it with its own rules (recommended), or upload and make a world active, the panel sets every rule to Keep, so that world plays with what it has stored. Choose its rules again on the tab if you want to change them.