Android operating systems are upgraded on a very regular basis, this is one of the reasons why people like Android phones so much. Most people actually keep waiting for new upgrades so that they can enjoy the perks of an upgraded system. After all the point of an upgrade is to get rid of annoying bugs and add new features.
Most users go berserk at the idea of a new upgrade as it is a great opportunity to try out the new things that app developers come up with every time there is a new update. But there are some people who would rather stick to the old version of Android. They would then need to learn how to downgrade marshmallow to lollipop.
Downgrading Android comes with its own set of problems, but some people still prefer doing it because sometimes the new versions have more glitches and bugs compared to old version. There might also be the issue of battery draining. Some apps are compatible with only older versions of the operating system and won’t run with the new one, so if you have any such apps and you absolutely need that app then the only way to keep using the app would be by downgrading.
You can learn how to downgrade marshmallow to lollipop easily enough from the internet. There are numerous tutorials, videos and blog posts that will guide you, but remember to back up all your data before downgrading so that you don’t lose them. Then all you need to do is locate an .apk file for lollipop and get it installed.
Custom ROM’s are also a good option if you don’t actually want to downgrade but still want the benefits of using an older version. You can easily find information about them in the Android forums.