What should be addressed first?
Many congregations have constitutions that are the Model Constitution for Congregations with the bare minimum changes necessary. But, as this site makes clear, there are many things that congregations can or should include in their constitutions. This "copy the Model and otherwise don't change anything" approach is unfortunate. The constitution, after the incorporation documents, is the highest legal document the congregation has. Incomplete or inaccurate constitutions can expose congregations to legal problems. The more well written and maintained the constitution is, the safer the congregation and its members will be should a problem ever arise.
Because there's so much to attend to in a constitution, congregations should approach them with consideration to the most important priorities. The list below is not absolute, but for congregations that have not cared for their governing documents in over a decade, this list is an excellent guide to prioritizing revisions.