It's impossible for Wikipedia to have a rule for every possible circumstance, so another rule exists requiring editors to find Consensus where written rules don't clearly apply.
Most of the time, agreement is worked out through the normal course of editing. After all, any edit made stands until it is challenged. But even this is subject to interpretation: multiple user essays on Wikipedia debate the relationship between "Silence and consensus".
Sometimes, when editors disagree, they bring it to discussion on "talk pages" and work out their differences. Of course, it doesn't always work like it's supposed to. And some topics lend themselves to consensus (mathematics) more easily than others (politics).