Does urine specific gravity increase or decrease in post-renal azotemia ? and what about pre renal ?

Depends : In pre renal azotemia if due dehydration or volume depletion, specific gravity should be high but depends on baseline kidney functions like any underlying kidney disease. I am not sure what you meant by post renal azotemia- in general someone passing lots of urine and is clear, specific gravity should be low as it is likely to be dilute urine; but are some exceptions.