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There are 4 main steps which need to be defined for updating eXo Platform extension from 3.0 to 3.5:
Update your project Maven dependencies in the POM files.
Update your eXo Platform 3.5 configurations.
Update components in your eXo Platform extension to use components of Platform 3.5
Update your eXo Platform extension which are customised features/components (were added new/ modified/ delete in eXo Platform 3.0 extension), and make sure they can work well in eXo Platform 3.5 environment.