You mean the in point and out point of a project? Go to the frame you want, and press "I" to set the i
n point. Or press "O" to set the o
ut point. Or, press "P" to auto-set the in and out points to the entire length of the p
If you mean the in or out point of a specific timeline object, you use the crop tool:
and drag either end to a new location.
You might benefit from giving the video tutorials a watch
(the Timeline one especially, in this case), or looking through the knowledgebase
, or reading the manual (The Essentials > Rendering), as all of them explain working with in and out points in different ways, as well as answering many other potential questions.