Instruction:
1. Download Xposed Installer (apk) and App Settings and install both of them.

2. Run Xposed Installer and press "Install/Update". Then also select the "Modules" tab, check the box to enable "App Settings". Then reboot.

3. Run "App Settings", select any app in the list you want to change to tablet mode. Change the value for the "DPI" and resolution (Screen (dp)) to the desired value, then press "Save", then Launch the program. Keep trying different values until it successful set the app into tablet mode.

Tips:
-If you are making change to an app that contain important data, use Carbon or Titanium Backup to backup your data first, just in case.

-I find "DPI" set to 200 or 240 and "resolution" to 1280x800 tend to work. Though not all apps come with a tablet mode.

-You can change the "font scale" value to make fonts bigger or smaller as well.

-If you experienced any app crashing, toggle it off in the App Settings menu and "Save". Also, setting the "DPI" to 0 is equivalent to the default DPI value.