At the moment, you can only Delete and recreate existing Shipping Rules. We will probably be adding an Edit feature in the new version of Seller Admin we've been working, possibly a Copy/Clone feature as well; I'll note that on the wishlist for good measure.
BTW, if you're adding a bunch of Rules at the same time, you may notice that after you Submit a new Rule, as long as you don't go back to Admin, that last-added Rule's settings remain in the Create New Rule section, so you can just change whatever setting(s) differ for the new rule and click Submit. E.g. if you're creating a set of identical rates for multiple countries, you'd probably want to add Rules for the first rate going country-by-country, then change the rate/thresholds and add Rules for that rate going country-by-country, etc.