How to replace a nuget package with a dll in a large solution

I need to make changes to a big (really big) solution.To achieve this, I have to change some code in a custom NuGet package used by the projects in the solution.

But I can't build a modified version of the package in the NuGet feed and then just update it in the solution.

I have access to the package's repo so I have a dll with the needed changes.

So, the issue is: how to replace the NuGet with this dll with minimum pain?When I tried to simply replace the needed NuGet's dll in the packages folder with the modified one, it caused the solution not to build.

Any help is appreciated.