Hello,
Yes, you to plan a downtime because need to reboot both devices.
Upgrade via CLI using TFTP server.
1. Download the software image where TFTP server installed.
2. Set the ip on switch and transfer the file to switch flash via TFTP server.
3. Set main image in both devices slot 1 and slot 2.
4. Once image is set then save the config and reboot the switch.
This will upgrade one by one.
You can refer pg. 15 from release notes for upgrade steps via tFTP server.
https://support.hpe.com/hpesc/public/docDisplay?docId=a00109606en_us
Similar way you can do it via GUI as well.
Device-->Upgrade-->View Software images
Go back to Device-->Upgrade-->Click on upgrade and upload the file into slot 1 and slot 2.
Both slot sould have same file and set new image as main image in boot-rom before reboot.
Thanks!