You can edit categories in Common Tasks by selecting Edit Categories under Settings.

You come to the editing screen.

View the existing categories with the category selector.

Move Task to a Different Category

To simply move a task from its current category to a different category, select the Category field and choose the category from the drop-down list.

You can create a new category if you wish. 

Click into the Category field and type the name of the new category rather than selecting an existing one from the drop-down list.

Add Task to an Additional Category

To add a task to an additional category, first duplicate it using Duplicate Record (the green copy icon). Then, change the category on the new record. You can change the category to an existing or new category following the instructions above.