There aren't any rules. Do whatever is most appropriate to your specific situation. What do the weights mean? That should probably tell you what weights to use in the new graph.
If there is no meaning, translation doesn't make sense: translation is, by definition, an operation that preserves meaning. For example, suppose I give you the following exercise:
Translate the program
function int int (hello_) repeat end begin int into Java.
The program doesn't mean anything so it can't be translated. You can't possibly give me a Java program and say "This is the right one" because there's no definition of "right". Similarly, without saying what the edges and weights of some digraph mean, there's no way to justify the claim "This undirected graph is the correct translation."
There's no intrinsic equivalence between directed and undirected graphs. Usually, one associates an undirected graph with the directed graph in which every edge is replaced by a directed edge in each direction. In many contexts, these behave the same way (e.g., if I can get from A to B in the graph, I can follow the same route in the digraph). In other circumstances, though, they might be different. For example, an undirected edge might represent the existence of a two-way road between two cities, whereas a pair of directed edges might represent separate one-way roads in each direction.