IOS 15.6.1 Can used for I phone SE
I phone SE software update for iOS 15.6.1 please forgive me
see: Update your iPhone, iPad, or iPod touch - Apple Support
And: If your iPhone, iPad, or iPod touch won't update - Apple Support
If the above Apple articles do not help, please post back with relevant information.
EDIT: I see you have quickly solved your tech support issue, well done.