6. Power off the phone and flash ONLY "BOOT_DELIVERY" from a Marshmallow firmware with Flashtool ( exclude all other partitions), you must only see
"boot_delivery.xml" in the "Content" column.
I have a question about this step. Where can I find this Marshmallow firmware?
I try to download another firmware is Marshmallow on the website "https://samsony.net/en/mobiles/F3215/download/1249"]https://samsony.net/en/mobiles/F3215/download/1249. But how can I put this image in flashtool that make me only flush "boot_delivery.xml" into my phone.
I supppose your phone model is F3215, and if not please find the correct firmware.
If your downloaded file is .zip, unzip it to the .ftf format.
Open flashtool and click the lightning button (i suppose your phone driver is ready for flashmode, fastboot and debug)
Browse to the location where you unzip your download.
It should load all the content, now click all exclude boxes until only "boot delivery.xml" left.
click all exclude boxes in TA also.
Flash.