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On any Star Alliance partner, you can redeem for Saver level awards on United via the partner site.
Avianca LifeMiles, Singapore, and Turkish are your options.
Avianca is probably the easiest to use (and I think the cheapest redemptions but search to be sure).
Note there is a $200 fee to get your points back if you cancel.
Make sure you check availability on the partner airline BEFORE transferring the miles!