For both types of phones - Android and iOS/IPhone - exists the same application: "NRF Toolbox".
It is free and downloadable from Apple Store / Google Store.
So - first you should install this app on your phone.
Function we will use for firmware update is "DFU" - click on DFU icon - and we have
main screen of DFU function.
Select file with firmware:
.., and we are on file selection screen.
If you can not see Droplet firmware files - on bottom is selector: "App files" and "User files" -> switch to "User files".
Select which firmware you wish to use:
when selected - click "Done"
and we are back to main DFU screen.
Next is selection of device:
and we have list of devices. Select - LimitterD (or LimitterD1, ...D2, etc):
When selected both file and device - "Upload" on main DFU screen is active.
Click - and firmware update process start...
Its takes few seconds - you can observe progress:
When finished - click OK.
Thats all. New firmware is working.
In applications - Spike and xDrip - click Forget then find - you can observe name change.
Each version of firmware have own number. Initial is "LimiTTerD", then "LimiTTerD1", "...D2", etc.
You can also observe firmware update process on Droplet.
When you select device - on Droplet RED LED light turns on. When update is finished - turns off, then 3 short blinks signalize firmware start.