I need to increase a letter by one, i.e. get the next letter in alphabet.

For instance: input = 'd', I need to get 'e'. Or input = 's', I need 't'

Any existing function/dll for that?

If your input letter is in variable ch1 then you can do the following,

ch2 = chr$(asc(ch1)+1)

And ch2 will contain the next character according to the alphabet order which is the same as the order of the characters in the ASCII table.