I have updated my Mini by downloading the update from direct link to Kobo Firmware then Copy Paste the update to my Mini. The store affilliation did not change. This site does have threads on how to change store affilliation. You can also open your Mini copy internal uSD card to new larger up to 32GB card that you install into you Mini. There are threads telling you how to do this. You can then keep original uSD card as backup copy. I think Kobo Mini is easy to open and reassemble but I have been taking things apart and sometimes putting them back together before I was 8 years old.
Direct Links to Kobo Firmware
https://www.mobileread.com/forums/sho...d.php?t=185660