Help Android Update from another model

If you want to flash a newer stock ROM to your phone, just be sure to back up your personal data before you do anything. And don't just assume everything is backed up, confirm the data is where you copied it to. Also note that with a Samsung stock ROM you don't need to root your phone or anything like that, that's only necessary if you're switching to a custom ROM.

As for specific instructions, if you go to the sammobile site for your model:
https://www.sammobile.com/firmwares/galaxy-alpha/SM-G850M/
select the proper one from the listing that matches your country and carrier. Don't substitute, ROMs are very specific and must match the hardware built into your model of phone, there's no mix-and-matching. At the bottom of the actual download page of the one you selected, there are basic instructions on what to do. An example being:
"Instructions

• Extract (unzip) the firmware file
• Download Odin v3.12.7
• Extract Odin ZIP file
• Open Odin v3.12.7
• Reboot Phone in Download Mode (press and hold Home + Power + Volume Down buttons)
• Connect phone and wait until you get a blue sign in Odin
• Add the firmware file to AP / PDA
• Make sure re-partition is NOT ticked
• Click the start button, sit back and wait few minutes"

One thing that might be an issue is their instructions refer to using the latest version of Odin. You might encounter problems with that as Odin is not very backwards compatible so in your case since this involves older hardware you might be better off using an older version (3.10.7). Here's a site with all versions in case you need it:
http://www.droidviews.com/download-odin-tool-for-samsung-galaxy-devices-all-versions/
If you don't have access to a Windows PC (Odin is a Windows utility), there's also Heimdall, and Open Source equivalent to Odin that's available for Linux, Mac, and Windows:
http://glassechidna.com.au/heimdall/

Also, note that sammobile intentionally restricts the speed of its download servers so even if you have a fast connection on your end, do not be surprised the download takes a while. You might instead want to use the updato site. Like sammobile, the downloads are verified as stock Samsung ROMs:
https://www.sammobile.com/firmwares/galaxy-alpha/SM-G850M/TCE/download/G850MUBS2CQA1/128113/