To be able to update a Salesforce object from Slack, it must be searchable from Slack (configured above). Once the Search parameters are configured, you can click on the link (shown below) to make it updatable.
You can further configure fields that may be updatable from Slack for that object. In this example, three fields have been selected for Opportunity.
Please also note that each object selected here must also have a layout defined in the previous step to allow Slack users to retrieve relevant information pertaining to that object via their search.