You can look in the TFS Admin Console to see if it is editable there but I don't remember it being editable in TFS. Not sure about the Azure DevOps rebrand version. The alternative is to use TfsConfig. Note that you'll need to remap the warehouse and reporting DBs as well if you're using that.
For the connection string you need to include the port number. For this the syntax is a little different <server>,<port>
(e.g. myserver,1234
). Also note that when you stop using the default port then SQL instances are no longer relevant. Each SQL instance requires its own port and therefore by specifying the port explicitly you don't need to include a SQL instance name anymore, if you were using them to begin with.