Issue:
- How to delete sketch constraints in Autodesk Fusion.
- Unable to modify sketch due to constraints.
Solution:
To achieve this follow the mentioned steps.
- Find the sketch from the browser.
- Go to Edit Sketch.
- Hover over a sketch constraint.
- Click Delete on your keyboard, or right click and click Delete.
Note: Clicking a sketch point or line could be needed for more constraints to appear. 
To select all the sketch constraints all together.
- Go to Select.
- Selection Filter > Turn off All Items, and select the Sketch Geometry constraints.
- Select the Window selection to select all the constraints.