There are several youtube videos about this. Just go to youtube then search for 'ford transit radio upgrade'. If you do this yourself, be sure to disconnect the battery before you start or you could accidentally trip something (like the airbag warning) and have to have a dealer reset it.
This one is a good one about removing the factory setup:
Personally, I would just go to a good auto sound shop, like B&B suggested, and pay someone to do it right.
If your rig doesn't already have a double din installed, you can get kits to upgrade to one, plus harnesses and antenna connectors, etc..
dash double din conversion kit for ford transit:
https://www.ebay.com/itm/Metra-99-58...RXrRej&vxp=mtr
wiring harness:
https://www.amazon.com/gp/product/B0...324ddbcfe3cefd
antenna adapter for ford transit:
https://www.amazon.com/gp/product/B0...ff410a2ed4a1ff