What is the best process to update windows 8.1 mobile to windows 10 mobile. Without even loosing any data And seamless experience on win10 mobile?
There really is only one way.. Download the windows insider app, register and pick a lane (fast or slow). Do a backup before doing this and restore it during setup
The best way (but could be an overkill) is:
-backup every media or content to onedrive or on an SD card
-hard reset
-install insider
-update x2
-hard reset again
-do not restore phone backup and reinstall apps from the "owned apps" list in the store
I did it this way and it is super smooth.
With the hard reset and fresh install you can not "restore" other settings.
But for msgs (SMS) you can. If you enabled SMS backup in w8.1 the SMS will turn up again in W10. The hard reset and clean install do not have an influence on the SMS "restore". You only need to use the same MS account in W10 and enable SMS backup.
Ps I am not sure what you mean with other settings but you may be able to sync some items with other devices you have with the same MS account.