You can install this specific version from NuGet.org or GitHub Packages using either of these commands:
Using the Package Manager Console:
PM> Install-Package MikaelDui.Net.UserAgent --Version 1.0.5
Using the .NET CLI:
dotnet add package MikaelDui.Net.UserAgent --version 1.0.5
But it's preferable to use floating versions, see the Microsoft Docs.