You can change your code to use the latest version (surely that won't be hard? it's only logging). Or if the assemblies are compatible you can use assembly redirection to have your code load the 1.2.13 assembly even though it is referencing an older version
Redirecting Assembly Versions[
^]
As I said though, that will only work if the assemblies are backward compatible.
The third option is to use both assemblies. This is possible, but awkward. You might need to google for more detailed info but if you look at the properties of the reference in VS you’ll see an "Aliases" field that will be "global". In your reference to the older version change that Aliases field to something like "oldlog4net". Now in your code anything that references "log4net.SomeClass" will use the later version, and you change your code to reference "oldlog4net::log4net.SomeClass" and that will use the assembly you referenced with the "oldlog4net" alias.