Hello alexmlc,
1. … look at Post #2, what Phoenix48 has written. Then download HxD (hex editor tool).
2. Microcode depends on your CPU - which one do you have?? Look on cpu-world for the CPUID (p.e. FX-8350 → 600F20).
3. Search & download AMD microcode here → p.e. cpu00600F20_ver0600084F_2016-01-25_C8460314.bin.
4. Download the latest (newest) mainboard-BIOS → which ons do you have (manufacturer + exact model + revision number)?? … and extract it on your HDD/SSD.
5. Download MCE and latest MCE-database (source code .zip). Then extract p.e. r60.zip in the folder of MCE.
6. Open MCE and copy’n’paste the mainboard BIOS into it, highlight the MCE-window & hit “Enter”. You’ll find the new folder [Extract]->[AMD]-> open old microcode, beginning with p.e. cpu00600F20_ver… into HxD.
7. Open BIOS-file with HxD, open new microcode (mc) into HxD. Mark all with [Ctrl]+[A] … watch for length (p.e. “A20” hex).
8. Mark+Copy the first line of the old mc, go tab of BIOS-file → [Ctrl]+[F] and search for “hex” → [Ctrl]+[V]. You’ll see the first line of the old mc.
9. In the BIOS-file mark manually from the beginning of this first line … to the end of length → “A20”.
10. Then go to tab of the new mc. Mark all, copy it … and insert it into the marked part of the BIOS-file. Save now!
11. Use MMTool v5.02.0024_patched (or MMTool v4.50.0023) to open the original BIOS-file → search for GUID of the microcode module → p.e. on my Sapphire’s board it’s called “AmdProcessorInitPeim”. If you don’t find it, ask here for help.
12. Look for the GUID behind → p.e. my is “DE3E049C-A218-4891-8658-5FC0FA84C788”. Remember it.
13. Go to UEFITool v0.22.4 and open it. Open the new edited BIOS-file with it. Search for the GUID above … and right-klick on it → Rebuild it.
14. [Ctrl]+[F] or go to “File”-Menu → “Save Image file”.
Now sending some €$€$€ or giving thanx to me … for writing this wonderful guide for you! 
Best regards, MiMo
P.S.
There are many ways to do it:
Intel, AMD & VIA CPU Microcode Repositories Discussion Topic
[Guru3D] Windows: How to get latest CPU microcode without modding the BIOS
AMD microcode