It is looking very poorly. I can't tell what is wrong, but you could try pruning the dead branches off, if they are dead, and I would take it out of the pot and check the roots. If they are rotten then it beyond rescue. Try not to disturb the roots if you can, just have a look.
If the roots are white and firm then they are ok, but I would not re pot unless the soil is not draining well.
Let us know,
Geoff.