As far as I know, you can't change that behavior. Editing view is the default, when you open a document for which you have editing rights.
Originally, Word Online did open documents in read mode. This was changed based on user request in the Uservoice forum. More recently, there have been suggestions in the Microsoft Community that the setting should be configurable, for example by the SharePoint administrator (see
https://answers.microsoft.com/en-us/...0-9ab36f298c99).