https://pgaskin.net/KoboStuff/kobofirmware.html
After you download the firmware for your device, extract the zip file into the .kobo folder on your Kobo eReader.
Release Notes 4.15.12920
Parental controls:
You can set a 4-digit PIN that must be entered to access the store and web browser on your eReader.
To turn it on, go to Settings and select Accounts.
Indented table of contents:
If your book's table of contents contains multiple levels, it will be indented for better readability.
Search improvements:
Typing is now more responsive when searching.
Wi Fi now reconnects more quickly before you search the store, allowing search suggestions to appear as you type.
Fixed:
Previously, when reading .MOBI books, your progress was not remembered.