This example will use Johnston Crescent, Western Sydney.
Note the appearance of roads under construction. They have orange-white banding along the road segment when you are in OSM editing mode - after clicking the Edit button in OSM. The tags they contain are:
construction=road classification i.e. Key:highway - OpenStreetMap Wiki. This example uses construction=unclassified
highway=construction
Example of a road under construction with the highway=construction tag.
With the under-construction road segment selected. Locate the Tags panel in the Edit feature pane. Note that this example uses construction=minor and conjunction with highway=construction
Replace these tags with highway=unclassified. If you also want to add a road name you can add name=Name of the road. More discussion of modifying road names can be found in Correcting/Changing Road Names
You can verify that the road is now opened as it carrying the symbology (the look and style) of the road classification added into the Tag panel in Step 3. Following this, you can save the edit, including a changeset comment