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not in the UI.

But the facet definitions can be found in \Files\System\Repositories - there are some somefolders named after the name of your repository and facets. You will have a file called Ecommerce.facets (Name made by you). Inside that file is some XML that will look something like this:

<?xml version="1.0" encoding="utf-8"?>
<Facets>
  <Settings />
  <Source Repository="Ecommerce" Item="All products.query" />
  <Facet Name="Wheel size" Type="Field" Field="ProductCategory_Bikes_Wheel_size" QueryParameter="wheel" />
  <Facet Name="Color" Type="Field" Field="VariantGroup_VARGRP1" QueryParameter="color" />
  <Facet Name="Manufacturer" Type="Field" Field="ManufacturerID" QueryParameter="manufacturer" />
  <Facet Name="Prices" Type="Field" Field="PriceGroups" QueryParameter="PriceGroups" />
  <Facet Name="Wheel size" Type="Field" Field="ProductCategory_Bikes_Wheel_size" QueryParameter="wheel" />
</Facets>

And then you can just move the Facet nodes up and down to change the sorting.