The software upgrade on the DXP is done by replacing hardware, a daughter board on the DXCPU, called DXPSW (rel). Additionally, this daughter board is mounted on top of another daughter board, DXRAM (usually 68K). To do the upgrade, you need these boards.
Saving the database is done (on releases to 9B) by using a communications program, such as Procomm or Hyperterminal. After the database is downloaded, it must be "massaged" by retrieving this database into VMMI, a proprietary communications program distributed by Comdial. The procedure is not unusually difficult if one reads about the steps in VMMI help screens.
Your biggest problem will be to obtain the new software and necessary ram. These are getting rare, because Comdial has elected to discontinue supporting the DXP, and the race is on to upgrade existing DXP's to later software. One possible supplier is King Technologies.
If you get to this poing, contact me, and I will advise you further.