I am wife already with Virkas but I now like Lydia more who can I change the plans or can't I?
Translation: "I'm already married with Vilkas, but now I like Lydia more. What can I do to change that?"
As far as I know, there's no way to get married to another NPC after the marriage is complete (including killing your spouse). But I may be wrong about that.
Of course, you can always load an older savegame...