To update the Forge version in LiquidBounce Legacy, first download the latest recommended version of Forge from the Minecraft Forge website. Run the Forge installer, select "Install client," and complete the installation. Then, download the latest version of LiquidBounce Legacy from the LiquidBounce website. Open your Minecraft launcher, create a new profile using the newly installed Forge version, and run Minecraft once with this profile to generate the necessary files. Close Minecraft afterward.
Next, navigate to your Minecraft directory (typically found at C:\Users\[YourUsername]\AppData\Roaming\.minecraft on Windows, or ~/Library/Application Support/minecraft on macOS) and open or create the mods folder. Move the downloaded LiquidBounce Legacy .jar file into this folder. Open your Minecraft launcher again, select the profile with the updated Forge version, and start the game. LiquidBounce Legacy should now be loaded with the updated Forge version. If you encounter issues, ensure compatibility between all versions and check for conflicts with other mods.