Installing by manually editing manifest.json

1. Read the instructions from the official Unity documentation here.

2. Open up manifest.json inside the Packages directory in your Unity project using a text editor.

3. Under the dependencies section of this file, you should add the following line at the top: "com.whinarn.unitymeshsimplifier": "https://github.com/Whinarn/UnityMeshSimplifier.git",

4. You should now see something like this (plus plenty of more packages):

{
  "dependencies": {
    "com.whinarn.unitymeshsimplifier": "https://github.com/Whinarn/UnityMeshSimplifier.git",
    "com.unity.burst": "1.0.4",
    "com.unity.mathematics": "1.0.1",
    "com.unity.package-manager-ui": "2.1.2"
  }
}

5. You can also specify to use a specific version of UnityMeshSimplifier if you wish by appending # to the Git URL followed by the package version. For example: "com.whinarn.unitymeshsimplifier": "https://github.com/Whinarn/UnityMeshSimplifier.git#v2.2.0",

6. Success! Start up Unity with your Unity project and you should see UnityMeshSimplifier appear in the Unity Package Manager.