You are already on beta ... run update, reboot and you should see "Switch" "Switch to alternative kernels" there.
Edit:
I just tested switching and it works fine, logs: http://ix.io/19ob
| ____|___ _ __ _ __ ___ ___ ___ ___ | |__ (_)_ __
| _| / __| '_ \| '__/ _ \/ __/ __|/ _ \| '_ \| | '_ \
| |___\__ \ |_) | | | __/\__ \__ \ (_) | |_) | | | | |
|_____|___/ .__/|_| \___||___/___/\___/|_.__/|_|_| |_|
|_|
Welcome to ARMBIAN 5.43.180428 nightly Ubuntu 18.04 LTS 4.16.5-mvebu64
System load: 0.18 0.30 0.13 Up time: 3 min
Memory usage: 6 % of 993MB IP: 172.16.100.189
Usage of /: 8% of 15G
[ 0 security updates available, 1 updates total: apt upgrade ]
Last check: 2018-05-04 08:36
Last login: Fri May 4 08:36:52 2018 from 172.16.100.175