change .net sdk code example

Example 1: how to change dotnet version

dotnet new globaljson --sdk-version [Type version number here] --force

version number examples:
- 2.1.509
- 3.0.100-preview-010184

Example 2: how to change the default version dotnet version

{
  "sdk": {
    "version": "2.1.103"
  }
}