Modrinth App

• Open the instance
• Click on "Instance settings"
• Under "Java and memory" you can set a custom memory allocation

Prism Launcher

• Select instance and click "Edit"
• Click on "Settings" tab
• Under "Java", check the Memory checkbox and set the Memory Usage

Minecraft Launcher

• Edit the installation
• Click on "More Options"
• Change the numbers in the JVM Arguments
  • Example: -Xms2G -Xmx4G = starts with 2GB, maximum 4GB