Here’s what you should do:

1. Make sure you have a memory card in your Symbian^3 device , you cannot change fonts using the Mass memory.

2. Then get any True type font with .ttf as extension (if you want Nokia Pure, click here ) .

3. Now navigate to the Resource folder in memory card’s Root (for example if F: is memory card , navigate to F:/Resource , if its not there, create one)

4. Now create a Folder called Fonts in F:/Resource (So the folder path should be F:/Resource/Fonts)

5. Now this is the most important step :

Make four copies of the font you have and rename the same font to the corresponding filenames. (for example: if you have helvetica(01).ttf rename it to nosnr60.ttf , like wise rename others to respective filenames given below)

6. After you have renamed to the above filenames, Copy them to the Fonts folder In F:/Resource/Fonts.

Now reboot the phone and enjoy your new fonts 

I have used the Helvetica Neue font and it looks like this: