Open the Minecraft Launcher
Go to the Installations tab
Turn on Version History if needed
Click New Installation
Open the Version dropdown
Select the older Minecraft version you want
Save the installation
Click Play on that installation
Install the correct Java version if the version requires it
Back up your worlds before opening them in an older version
Use a separate game directory for older versions if needed
Install mods or Forge only if the version supports them
Use the official launcher or a trusted launcher
Avoid opening newer worlds in much older versions
Check that your account is signed in
Make sure the version files finish downloading before launching