The convert pen tool in Photoshop can be found beneath the pen tool in the tools panel of the application, unless you have customized the app and the tools.
You can use the tool to change a point from a smooth to a corner or manipulated the corner back into a smooth by changing the direction and direction lines for the current active anchor.
Select the tool, go to the point and click. If it is a smooth (curved line) then it will be turned into a corner (straight lines) . If it is a corner then you can drag out the direction lines etc using the tool and create a curved smooth. By default, the direction lines will be the same length but you can use the tool to select either the left or right lines and manipulate them separately.
You can also hold down the cmd / ctrl key on the keyboard and the result will the direction tool is activated and that can then be used to move the anchor.
There is no additional features or options for the feature but if you hold down the shift key then the lines will be constrained to 45 degrees, 90 degrees etc.
There is no special way to turn the entire path into a straight line design, you will need to click each for that conversion. Please note that this tool works on vector designs and not raster designs and that it works on your own vector shapes, lines, custom shapes as well as other shapes generated.