I would like to change the titles: "important, business, vacation" etc. to titles that have meaning to me.
How can I do so ?
You can edit the labels from the tool bar when in the main calendar view.
Each calender can have it's own set of labels, so open the calender you want first then edit.
Here's a screenshot for you:
PS: It's all greyed out in the pic because I haven't got an appointment selected, if you did you could quickly switch the label as well without opening the appointment.