If you’re on an older version than 4.1 please follow previous upgrade guides and perform those upgrades incrementally**, eg.
  1. upgrade 3.7 to 4.0
  2. upgrade 4.0 to 4.1
  3. upgrade 4.1 to 4.2

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Update Gemfile

Since Spree 4.3 spree gem doesn’t include spree_emails, spree_backend and spree_frontend gems. It only consists of Spree Core and Spree API. You need to add them to your Gemfile if you want to use them. 
bundle add spree_emails spree_backend spree_frontend

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Remove gems merged into Spree Core

These gems were merged into Spree Core in version 4.3. All of their functionality is now included in the spree gem so you can remove them.

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Optional Remove SpreeMultiDomain

If you used that gem in the past you need to remove it. Multi-Store is now incorporated into Spree core and you cannot use that gem anymore.
  1. Remove spree_multi_domain from your Gemfile 
    bundle remove spree_multi_domain
  2. Remove //= require spree/frontend/spree_multi_domain from vendor/assets/javascripts/spree/frontend/all.js
  3. Remove //= require spree/backend/spree_multi_domain from vendor/assets/javascripts/spree/backend/all.js

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Optional Remove Spree Editor

Spree 4.3 includes TinyMCE 5 editor built-in. It’s not recommended to use spree_editor gem anymore. 
bundle remove spree_editor

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Optional Remove Spree Static Content

Spree 4.3 includes a built-in CMS. It’s not recommended to use spree_static_content 
bundle remove spree_static_content

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Install missing migrations

rake railties:install:migrations

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Run migrations

bin/rails db:migrate

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Additional fixes and hints

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Upgrade Sprockets to v4

In your project create app/assets/config/manifest.jss file with contents: 
//= link_tree ../images
//= link_tree ../javascripts
//= link_directory ../stylesheets .css
More on this topic.

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Admin Panel fix

If you’ve developed custom features for your Admin Panel, please replace any occurrences of 
render partial: 'spree/shared/error_messages'
to 
render partial: 'spree/admin/shared/error_messages'

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Read the release notes

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