If you have gacutil, you can install an assembly to the GAC via:

gacutil /i mydll.dll

Or disable cache viewer and manually updating the file:

https://www.symantec.com/connect/blogs/using-windows-explorer-view-global-assembly-catalog

Sometimes fix/patch will require .NET 4GAC. In that case, the following steps might help.

Normal GAC Location
C:Windowsassembly
-----------------------------------------
.NET 4GAC Location
C:WindowsMicrosoft.NETassemblyGAC_MSIL

Procedure to GAC in the .NET 4 GAC

1) Browse to C:WindowsMicrosoft.NETassemblyGAC_MSIL
2) Delete the existing file in the GAC (Folder and ALL) If this is not done the file WILL NOT be updated.

3) Gac the new assembly using GACUtil or the installer tools. This files are part of the installer Source files and installed files.

4) Drag and drop your assembly on the “SourceCode.Configuration.GAC.NET4.exe”
5) Verify that the file is added to the GAC.

I like how everyone is giving different answers and different guides, and so I will join in the fun.

I am also assuming you have gacutil. If you don't have gacutil, drop me a PM and I will give you a dropbox or onedrive link to download it.

Anyways, I always recommend people to uninstall the existing assembly in GAC first. For your case, run the following in command prompt from the gacutil folder:

  gacutil -u SourceCode

Note: don't put the .dll extension, just the name will do.

That should uninstall the assembly from GAC, and then, install with the following:

Assuming you have the SourceCode.dll in a folder named "Coldfixes" in C: drive,

  gacutil -i "C:ColdfixesSourceCode.dll" <- Path to file, and include the .dll extension

or you could put the coldfix file into the same folder as gacutil and just run,

  gacutil -i SourceCode.dll

That should install the assembly to GAC. Once done, just check the properties of the assembly in GAC and confirm that the size and details matches up with the coldfix and you should be golden.