You don't unless Skystream makes an update available. Contact Skystream, as they will be able to tell you with certainty whether an update is available or not, and how to install one if there is one available. That said, given the only product Skystream currently sells is a Pro 8K Media Player, and given that Media Player uses the Google Assistant, meaning it's running at least Android 7, your device is likely too old and no longer supported.
Anything is possible, though you don't need root access to replace the firmware. What you DO need is an unlocked bootloader, a custom recovery, and a ROM image. I don't know if any exist, meaning you would have to create them.
No idea how to do it on your device. You could try XDA Developers and see if on their forums someone encountered a similar issue, but keep in mind, without a custom ROM and a custom recovery, unlocking the bootloader will not do you any good.
Procedure to upgrade device OS from KitKat to Lollipop:
Download OS recovery package file, M18N0LXXXRUXX20902.zip from Support Portal: MC18 Operating System - Android Support & Downloads | Zebra
Keep the file in .zip format.
Connect device to PC via USB port. Note: PC must support MTP.
Make sure MTP mode is selected in the device's USB setting and MTP driver installed successfully in PC.
The internal SD card of the MC18 will be seen on your PC as mounted drive MC18N0.
Copy " M18N0LXXXRUxx20902.zip" file to the root folder of “On Device Storage”.
Remove USB cable from PC and keep the device in cradle with power supply.
Press and hold scan button for approximately 10 seconds, now the device will be in power/switch off mode.
Press the scan button briefly (2 secs) and release it. The device will start to power on.
Keep holding down scan trigger immediately after the devices powers on in prior step.
When Android logo appears on the screen release scan trigger. Note: If the terminal is in cradle and the trigger is pressed for more than 10 seconds it will power off. So, in step above, ensure to remove the trigger as soon as the android logo appears.
Navigation in the recovery menu is as follows:
Tap on the display to switch through the options/files.
Press scan button to select an option/file.
Select "apply update from On Device Storage"
Navigate and select M18N0LXXXRUxx20902.zip on /sdcard.
When update is complete, select "reboot system now"
Note: Battery must be 30% charged to perform OS update.