You need to Magento_LayeredNavigation/templates/layer/view.phtml to your theme folder.
Now change,
data-mage-init='{"accordion":{"openedState": "active", "collapsible": true, "active": false, "multipleCollapsible": false}}'
with,
data-mage-init='{"accordion":{"openedState": "active", "collapsible": true, "active": [0,1,2,3,4,5,6], "multipleCollapsible": false}}'
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