If you need to troubleshoot a specific project migration error, please share: The exact you are using. The error message text or error code you are encountering.
For Visual Studio 2012 and newer, the .NET Framework 4.5 targeting pack is typically included out of the box. net framework 45 developer pack link
If you need to deploy the runtime to machines without internet access, look for the ".NET Framework 4.5 Offline Installer" on the Microsoft Download Center. What is the .NET Framework 4.5 Developer Pack? If you need to troubleshoot a specific project
.NET 4.5 is "in-place," meaning 4.8 will usually run 4.5 code, but you still need the 4.5 SDK to build it specifically. net framework 45 developer pack link