Which spelling is correct: "Re-order" or "Reorder"
Reorder.
With re- words, you should use ‘re-’ (with a hyphen) if the next word begins with an ‘e’ or a ‘u’ (when not pronounced like ‘you’). Otherwise, don’t hyphenate. It’s therefore re-examine, re-urge, re-entry and re-elect, and reuse, reunion, reorder, reinforce and redevelop.
Source: http://www.proofreadinglondon.com/blog/to-hyphenate-or-not-to-hyphenate
In general, try to avoid putting hyphens into words formed of one word and a short prefix
Source: http://www.economist.com/style-guide/hyphens