Q:

An existing package by the same name with a conflicting signature is already installed

I'm trying to update (Documents to go Main app) in my device.
When I'm trying to install it an error comes out says an existing package by the same name with a conflicting signature is already installed.
I have root access on my device. Is there anything I can do?

A:

First, if there's a signature mismatch and I'm sure the original app comes from a trusted source, I'd doubt the source the "update" comes from: the .apk very likely is manipulated, and might include malware -- so be warned! I do not know the "one mobile market" you've downloaded it from, so I cannot tell for sure.
If you still want to install the new .apk: Yes, removing (uninstalling) the previously installed app and the installing the downloaded version should work. If the first one was installed as a system app, you will need the appropriate system tools to uninstall it (e.g. Titanium Backup  -- which you then could also use to create a backup before removing the app, just in case). Alternatively, if you know the right command line tools, you can do it also via adb shell.
As the app in question does not require to be made a system app, installation can be done normally without any root stuff: just tap the .apk in your file explorer, and let the normal installer do its job.
After that, watch out for any strange behavior -- just in case the .apk introduced malware.

