You can easily turn a sub-activity to a top level activity.

Method 1 (drag-n-drop):

1. Open Activity Center.
2. Select the hierarchical view mode.
3. Locate the activity you want to turn into a top-level activity.
4. Drag the activity onto the project row until the latter gets surrounded with a thick black rectangular frame..
5. Let go of the activity being moved.

This method works only in hierarchical view mode.

If displaying projects is off (the Projects checkbox is not selected on the customization panel), this method works only within the project the activity belongs to.

Method 2:

1. Open Activity Center.
2. Select the activity you want to turn into a sub-activity of another activity.
3. In Activity details section, hover the pointer over the hierarchy of the activity until '— change' appears.
4. Click '— change'.
5. Click 'set as top-level activity'.