This is simple Application to change DPI on the fly. Though some app may requires to clear data to adopt new DPI. Also very few app needs to reboot to apply changes.
This app supports 240-650 DPI to set from.
Requirements :
- Needs rooted device.

How to use.
Open application and select your DPI from DPI picker.
Selct 'SET DPI'
Done.

Sometimes setting certain DPI may makes many applications non usable. And when your launcher / keyboard ecomes unusable then it will be difficult to reset. So when you testing DPI it's recommended to select "Auto Reser' option ON. This will reset DPI to normal on reboot in case device becomes unusable to perform action.

Also to note that this application won't make any application multi DPI, this will depends on your ROM.

There is option for reset DPI anytime from menu.