Easy way. Use the system transfer option in the system settings, to copy over all stuff from o2DS to New2DS XL. Nintendo has a few support site especially for it. U can use the PC, or move direct system to system over Wlan.
To delete everything on the old one, u can use the restore default settings option in system settings.
Important note: do not register your Nintendo Network ID (NNID) on your New Nintendo 3DS system prior to the system transfer. Your NNID will be automatically transferred along with your data. Registering your NNID on the target system beforehand will make data transfer impossible.