Q: how do i change my apn?

Try a search of the forums or ask Straighttalk.

They are not an authorised or approved carrier, as such, they would need to provide the information.

Please be aware that this link has not been tested by me. YMMV.

http://aaltonen.us/2012/02/07/howto-straight-talk-iphone/

or

http://support.apple.com/kb/HT2283

Go to unlockit.co.nz from your phone (on wifi) and select Straight Talk. You will be able to apply a profile to your phone that sets the correct "att.mvno" cellular data APN.