Raspberry Pi 3 in nextcloud box - firmware update with snap

Nextcloud version: 11.0.5
Operating system and version: Ubuntu Core
PHP version:7.0.23

Hi, can somebody please tell me, how to check and update my Raspberry firmware?
Before on my rpi2 I used rpi-update (installed with apt).
As Ubuntu core does not support apt, I have to use snap?

thx

Got it running this way:

root@nextcloud:/home/shadowempire# snap install classic --devmode --edge
root@nextcloud:/home/shadowempire# classic
(classic)shadowempire@nextcloud:~$ sudo apt-get install wget
(classic)shadowempire@nextcloud:~$ wget https://github.com/Hexxeh/rpi-update/raw/master/rpi-update
(classic)shadowempire@nextcloud:~$ sudo apt-get install rpi-update
(classic)shadowempire@nextcloud:~$ sudo chmod +x rpi-update
(classic)shadowempire@nextcloud:~$ sudo apt-get install curl
(classic)shadowempire@nextcloud:~$ sudo apt-get install elfutils
(classic)shadowempire@nextcloud:~$ sudo apt-get install binutils
(classic)shadowempire@nextcloud:~$ sudo ./rpi-update
*** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
*** Performing self-update
*** Relaunching after update
*** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
*** We’re running for the first time
*** Backing up files (this will take a few minutes)
*** Backing up firmware
*** Backing up modules 4.4.0-1030-raspi2
#############################################################
This update bumps to rpi-4.9.y linux tree
Be aware there could be compatibility issues with some drivers
Discussion here:
https://www.raspberrypi.org/forums/viewtopic.php?f=29&t=167934
##############################################################
*** Downloading specific firmware revision (this will take a few minutes)
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 168 0 168 0 0 153 0 --:–:-- 0:00:01 --:–:-- 153
100 54.1M 100 54.1M 0 0 940k 0 0:00:58 0:00:58 --:–:-- 1217k
*** Updating firmware
*** Updating kernel modules
*** depmod 4.9.65+
*** depmod 4.9.65-v7+
*** Updating VideoCore libraries
*** Using HardFP libraries
*** Updating SDK
*** Running ldconfig
*** Storing current firmware revision
*** Deleting downloaded files
*** Syncing changes to disk
*** If no errors appeared, your firmware was successfully updated to e4b56bb7efe47319e9478cfc577647e51c48e909
*** A reboot is needed to activate the new firmware

Check for updates after the first installation/reboot:

root@nextcloud:/home/shadowempire# classic
(classic)shadowempire@nextcloud:~$ sudo su
root@nextcloud:/home/shadowempire# ./rpi-update
*** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
*** Performing self-update
*** Relaunching after update
*** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
*** Your firmware is already up to date