Also, "EPiServer.CommonFramework" assembly is not present in CMS v10 . What is the relacement for this assembly?

@run k,

This article explains how to remove the dependency on the Common Framework: https://world.episerver.com/documentation/upgrading/episerver-commerce/8/Breaking-changes-Commerce-8/Removing-dependency-on-Common-Framework/. That article mentions "If you use any functionality from Common Framework, you need to install the EPiServer.CommonFramework package, as well as the EPiServer.Common.Gadgets package."

When i am trying to install  EPiServer.CommonFramework  then i am getting .NETFramework error for this packge to be installed.

Below is the error messge : 

 Could not install package 'EPiServer.Mail 9.0.0'. You are trying to install this package into a project that targets '.NETFramework,Version=v4.5', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.

Do i need to first downgrade .NETFramework and install "EPiServer.CommonFramework"  and "EPiServer.Common.Gadgets" and then do rest of upgrade to Episerver v10.

EPiServer.CommonFramework is legacy => this is true. But actually CommonFramework 8.0.0 is CMS10 compatible.

I'd suggest to change the .NET framework to 4.5, install CommonFramework (and EPiServer.Common.Gadgets), then change to .NET 4.5.2 and install Mail.