Unfortunately Metro PCS users have no luxury to have that. As usual Metro PCS's software update is no-exists.
But fortunately most of newer phones that came out after T-Mobile and Metro PCS merger are same phone with only cosmetic differences. They are same phones except whether you have T-mobile branded apps & icons or Metro branded ones. Thus (most of the time) you can safely install T-mobile ROM to Metro and vice versa.
Once you install T-mobile software to Metro PCS phone and make it to be recognized it as T-mobile phone. You will be able to download Marshmallow software update.
Let me provide you step by step to do so. I am assuming you know how to root and how to make back up / restore. I am not responsible for bricking your phone or any damage done to yours.
1) Wifi connection
T-mobile sends update through Wifi, even if you have strong 4G LTE signal you can't install MM without Wi-fi connection.
2) Enough Battery on phone at least 25% - 30%
T-mobile web site recommends at least 50% though.
3) T-mobile KDZ file
To go back to T-mobile stock
4) LG Flash Tool 2014
To Flash T-mobile KDZ
5) Backup all important files to PC or external SD card.
6) USB cable, computer, backup/restore/stock files in case anything goes bad.
Go back to T-mobile Stock (H63110I)
1. First check this unbrick guide from GameTheory
2. Download T-mobile 10j KDZ file. If 10J is not available use 10I instead.
T-mobile 10i KDZ link
3. Download LG Flashtool 2014
4. Follow the unbrick guide and install T-mobile 10i KDZ to your phone. Your Stylo has to be either T-mobile or Metro PCS brand.
6. Now you have T-mobile LG G Stylo.
7. Set up phone as usual; check internet connection, connect to wifi, sign up
Hello Guy Fieri :) What a screenshot....
8. Go to Settings -> General -> Date & Time -> Uncheck Auto-date & time.
9. Manually set time 11:30 PM. Hit OK then reboot
10. Setting -> General -> About Phone -> Update Center -> System Updates.
11. You should see there's Android System Update.
12. Download the update. While downloading, do not unlock developer option or apply root, this may permanently block MM update. The only update on phone should be this System update and couple of automatic app updates.
Again, just patiently wait until download finishes.
13. Once download finishes apply the patch. Phone will reboot to recover session and install the patch automatically.
14. There should be total 2-3 minor patches until you see 700MB Android 6.0 update. If you don't see those patches, repeat step 9; reset time 11:30 PM, reboot phone, check system update.
15. Once you install final patch (means you have software H63110L), you should see large update, almost 700mb and message saying T-mobile presents Android 6.0. This is the Marshmallow patch.
16. Download the patch and update.
17. Phone reboots. Now you have Marshmallow LG G Stylo.
Not much difference though...
Official T-mobile Page
GameTheory Stock Restore Guide
T-mobile Stylo 10I KDZ