As Salesforce upgrades from one release to another release, Salesforce introduces new versions of the API. For example, if the version of the metadata API release is 38 in the summer release, then in the winter release it will be version 39.
Flosum is always tied to a specific version of the metadata API to retrieve and deploy changes. For example, it will use version 38 of the API to pull and push changes to other orgs. This version is independent of your Salesforce release (for example, it does not matter if you are using Summer 2017 or winter 2018 release). Flosum will use the same version of the underlying API.
As Salesforce upgrades from one release to another (say summer release to winter release), Flosum uses the same underlying version of the API. As an example, your org may be upgraded from summer to winter release, but Flosum can use version 38 in both the cases.
So when you are upgrading the sandboxes or the main production org with the new Salesforce release, it does not really matter since Flosum is tied to a specific metadata API version, not to the main Salesforce release schedule.
From time-to-time, Flosum upgrades the version of the API that it leverages to retrieve/deploy code.