Can I improve the water pressure by changing from 1/2" to 3/4" pipe?
Do you have a pressure reducing shower head?  Increasing the size of the pipe will reduce friction losses and result in slightly more pressure - my impression is the effect wouldn't be large.  

How much more pressure are you hoping to get?  Do you know the water pressure for your house?  

Are your pipes currently copper or pex?  Switching to pex and avoiding 90s would probably do more to reduce friction than simply upsizing your pipe.