shall i consider only downstream(outlet) only or both(upstream and downstream).
That really depends on your project scope and the exact system you have. However, my guess is "both".
As per API-520, calculate force and apply for it.
Yes, you need to apply the thrust (relief) force to your model. This is an input item for
CAESAR II. If you can't compute this, ask your process people.
In downstream shall i give anchor support.
Well at some point you will hit a terminal point (some type of nozzel), since your pipe goes "some place". Terminal points are either anchors or predefined displacements. As to whether or not you have other intermediate anchors, that is "system specific".
how i give support for psv lines
Depending on the Code, you need to determine where to place supports for the "primary" and "secondary" loading condtions, as well as addressing any specific operating events (such as the valve firing).