Obsolete command \Bbb

Since some years the command \Bbb provided by amsfonts has been renamed to \mathbb to conform to the other font selection commands (\mathcal, \mathbf and so on).

The command \Bbb has so been redefined to give a warning at the first usage, but then it works exactly as \mathbb.

Of course you're free to redefine it as you please:

\usepackage{amssymb} % or amsfonts
\renewcommand{\Bbb}{\mathbb}

In this way you'll not get any warning.

Why hasn't it been removed? Because many legacy documents use it.

I suspect you are using this command all over the place to typeset something like a matrix or perhaps a set of numbers. You should try to separate style from content, while at the same time save your fingers from excessive typing.

\newcommand{\set}[1]{\mathbb{#1}}
...
$ x \in \set{N} $

Both short and readable, and, if ever necessary, simple to change styling (e.g. you wish to use some other font for sets).