What gets replaced and what stays
Updating swaps the modpack's bundled content, mods, configs, scripts, and resource packs that the pack ships with. Anything you generated by playing, saves (worlds), screenshots, server lists, options.txt, your own added mods or resource packs, stays in place.
If the new version removes a mod that was generating world data, that data won't disappear, but the world may show missing-block warnings or refuse to load until the mod is restored. Major version jumps (1.20 → 1.21 modpacks, for example) often introduce save-format changes.
Locked vs unlocked instances
By default, modpack instances are locked. Locked means GDLauncher manages the bundled mods for you and prevents accidental edits that would conflict with the pack's manifest. Updating works the same way whether locked or unlocked; the difference matters mostly when you want to add or remove individual mods between updates.
If you've unlocked an instance and added your own mods, those extra mods stay through the update, they're not part of the pack manifest, so the version swap leaves them alone.
When to update, and when not to
Update when the pack author releases a bug-fix or content update for the same Minecraft version, or when you're starting a new world and want the latest content anyway. Don't update mid-playthrough on a major version unless the patch notes explicitly mention save compatibility.
Big modpacks often include their own changelog under the version description. Skim it for words like "breaking", "world reset recommended", or "new mods". If you see those, back up the world folder first.
Backing up before an update
The safe move on any meaningful update is to copy the instance folder before changing versions. Right-click the instance → Open folder, then copy the entire folder somewhere outside GDLauncher's data directory. If the update breaks something, you can paste it back.
For just the world: open the instance folder, then saves, and copy the world folder you care about.
If something goes wrong
The same Change Modpack Version button lets you switch back to the previous release. As long as you didn't play far enough into the new version for save changes to be permanent, rolling back is usually clean.
If the launch crashes, check the launcher's per-instance log under <instance>/logs/ and Minecraft's own log under <instance>/instance/logs/latest.log. Crash dumps land in <instance>/instance/crash-reports/. Missing dependencies are a common cause and are often fixed by reinstalling the pack: Settings → Reinstall.