To modify an existing calendar, go to Enterprise > Calendars. Then click on the calendar to modify and hit 'Modify', and make the desired changes.
To assign a different calendar to an activity, click on that activty, go to the General tab in the Activity Details on the bottom of the screen. Under the General tab, on the right hand side, there is drop down menu for Activity Calendar. You can change the assigned calendar for that activity with this menu.
Hope this helps!
You can make the necessary changes in calender without affecting individual activities by going to the Projects tab and then choosing the defaults option. Here the calender menu is visible wherein you can opt for the desired calender to the entire project.