Here's a tutorial about how I did it, and how to change your fonts too:
Before you start you should know:
1) I'm not an expert, I just have some patience to try things out so I won't guarantee that this method will work with your device and chosen font.
2) If you have already installed a cusomized font via a cab file, uninstall it before copying any font files to your device.
3) You will not be able to copy and overwrite any manually copied font files. If you haven previously copied any font files over the ones in the ROM, you will need to rename these using a powerful file explorer such as Total Commander PPC. I recommend renaming the extension to .ttt then, after a soft reset, you can copy them onto your storage card and rename the extensions back to .ttf.
You'll need at least two fonts, one regular and one bold (or one light and one regular, etc) First make copies of your two fonts and rename the files:
respectively. Now, you will need some software called FontCreator. It's quite expensive so you might want to find a trial version of version 5.0 of the program as this one lets you save files. Open FontCreator and open tahoma.ttf. In the Tools menu select "AutoNaming..." and type "Tahoma" in the "Font Family Name:" field. Ensure the drop down box below the font family name field is "Regular" and click next, then Finish, then of course, save.
Follow this procedure for tahomabd.ttf except when you get into the "AutoNaming..." wizard again select "Tahoma" as the font family name but select "Bold" in the box underneath.
Make a new copies of your regular and bold fonts and rename the files:
1) HelveticaNeue LT 35 Thin.ttf
respectively. Open HelveticaNeue LT 35 Thin.ttf in FontCreator and again go to Tools and "AutoNaming...". This time put "HelveticaNeue LT 35 Thin" in the Font Family Name field and select regular then next, finish and save the font. Follow the same procedure for "HelveticaNeueOTS.ttf" but this time using "Helvetica Neue OTS" as the Font Family name.
Make another copy of your bold font and rename the file: "ZZ.TTF" Open ZZ.TTF in FontCreator and go to Tools and "AutoNaming...". This time, simply add " R" at the end of the Font Family Name. So for example "Comfortaa" will become "Comfortaa R". Finish the AutoNaming wizard and save the file.
You are now ready to copy files to your device. The following files go into "\Windows\":
3) HelveticaNeue LT 35 Thin.ttf
The following files to into "\Windows\Fonts\":
1) Your original regular font file.
2) Your original bold font file.
One point to note, you will not be able to replace files in the ROM of your device (the font files in \Windows\) via your PC. You will need to use a more powerful file explorer such as Total Commander PPC.
Soft reset your device after copying the files and your new fonts should be active.
To uninstall your font:
1) Rename the copied font files' extensions in Total Commander PPC to .ttt, do for example "tahoma.ttf" becomes "tahoma.ttt".
2) Soft reset the device.
3) Delete the .ttt files.