Go to Conversion > Upsell & Cross-sell > Collection.

The left panel displays a list of your store's collections. Click on a collection (llick on the item image or title itself, not the checkbox), and a panel with two tabs (Cross- & Upsell and Box Settings) will open up on the right side. Click on Cross- & Upsell.
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Click on the add (+) button. A window with all your collections will appear to the right of the interface.

Select the collection(s) you would like to upsell and/or cross-sell against the collection you previously selected.

Click on the Add selected items button.
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Once the intended collections are added, they will automatically be set up as both cross-sell and upsell. If you would like to restrict the relationship to either upsell or cross-sell, click the green arrow beside either of these selections to remove it.
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Once you are happy with your selections, click Save.
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