Kili Technology allows to easily draw bounding boxes. It is a rectangle to position on the object, the first click will determine the upper-left vertex of the rectangle, and the second click the lower-right vertex.
It is also always possible to change the class of an annotation with a left click, selecting "Change class", and then select the new class of the object. If the task is multi-classification, this operation will allow you to select all the categories for this annotation : you can delete the previous class but also just add one or more others.
Bounding box can be nested (see nested classification)) with all classification tasks, to build refined object classification.