you could just assign the letters as textures that dont change. another way is to script it, yet another way is to define it as another running number with different textures for the numbers...
for instance, youd have the regular digit 3,4,5 and 6 defined, and put digits 1,2 defined as letters (in this case you could only use 10 though).
if you are not a content creator you probably have no idea what any of that means. the easiest way would be to just have the letters on the main texture in front of where the running numbers go.