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Go to 'Mix Packs' in the 'My Music' section where you'll see three dots on the right of every style.
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Click on these dots to bring up a menu and select 'Remove Mix Pack'.
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This will uninstall the style but it should remain visible in the list.
Note: if you wish to reactivate this style, simple select it to down load it again (you will not be charged a second time for this mixpack).