Using the following command about global declaration for sizes, we can easily modify the margin of beamer slides:

\setbeamersize{text margin left=<size>,text margin right=<size>}

The following code example corresponds to a simple frame with a modified beamer margin (1.5cm left and 2cm right):

% Modify Beamer margin

% Theme choice

% Beamer margin
    text margin left=1.5cm,
    text margin right=2cm

% Dummy text


\begin{frame}{Modified margins (1.5cm left, 2cm right)}

Compiling this code yields the following:

Modified beamer margin globally
A frame with modified margins.

where default margins look as follows:

A Beamer frame with modified margins
A frame with normal margins.