The TabControl has a SelectedItem property which can be used to get or set the currently selected TabItem. Thus, you can use an int field to keep track of the currently selected tab, and in the Next button's event handler, just increase the int field by one and set the SelectedTab property to the corresponding index
currentIndex += 1;
myTabControl.SelectedTab = myTabControl.Items[currentIndex];
And vice versa for the Previous button.
Hope this helps