What do I so wen 31 week old child has a fever of 101.9 and she is teething and have runny nose?

Go to the doctor. Teething is rarely associated with a fever of more than 100.4°f in a young child. The cold may be a source of fever but the child should be assessed for other possibly more serious causes.
Call Pediatrician. The fever and the runny nose are most likely not related to the teething. At least, it needs to be ruled out as the cause. Call your pediatrician and ask for advice and whether or not he\she thinks that your child should be seen right away.