Fortigate 30E Backup device against Problem device how to

Hello, 

i have a Fortigate 30E in our enviroment, but i lost the login credentials. now i bought a second one of the Fortigate 30E. so i want to make a firmware update, but it seems to be its impossible to get firware without registrating the device in the portal of Fortinet. 

First question, if i register the device to get an update, can i unregister it, for selling it after using ist for my work? 

so if the firmware was updated, ill want to send it to the location for using it for the guest w-lan connection. 

the problem device with the lost login credentials would be send back to me, so i can make to reset the device an make a firmware update too, on it. 

if the problem fortigate 30E was reconfigured, ill want to chage it to the first fortigate 30E again, and want to delete the backup fortigate 30E. 

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second question, how can i make a reset on the problem fortigate 30E

how can i do that, is it possible to reset the configured fortigate 30E , reset, new configuration, is there a how to do for me, to make it work again? 

any ideas ? 

third question 

and last but not least, how ill configure the fortigate 30E to make an Internet provider login attached to a DSL Account from Deutsche Telekom? is there a how to to too for me? 

thanks for you support

Centipede1968