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Update gems

bundle update

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Install missing migrations

rake railties:install:migrations

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Run migrations

rails db:migrate

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Migrate Spree::Taxon icons to Spree Assets

We changed Spree::Taxon icon to use Spree::Asset to unify attachment usage across all Spree models. If you were using icon images in Spree::Taxon please run this to migrate your icons: 
rails db:migrate_taxon_icons

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Read the release notes

For information about changes contained within this release, please read the 3.4.0 Release Notes.