There is nothing wrong with the default font on Android. It is clean, crisp and clear. But, if you are kind of bored of that font, then changing it would surely give its interface a new look. If you are using Android 2.2 or higher version, then you would offered with a default option of changing fonts.
Change Default Font On Android Devices
Launch the Android market, and type the font that you want to download. So once you search for it over there, you will be displayed with number of fonts available in the Android market.
All you need to do is download the ones that you like. Or, there is an easier option. Search for Flipfont, in the market and you would be displayed with number of fonts. So, choose the ones what you like and download them on your Android.
Now, navigate your Android as Settings > Display > Font Style. There you will find the list of all the fonts that you have installed in the above step.
Now just choose the default font for your Android, and confirm the same when it asks. Now, it your restart your device to enable the changes.
That is it! This is how you can change default font on Android devices.