$ sudo apt-get update
$ sudo apt-get install libssh2-1 

libssh2-1 is installed in your system.

Make ensure the libssh2-1 package were installed using the commands given below,

$ sudo dpkg-query -l | grep libssh2-1 *

You will get with libssh2-1 package name, version, architecture and description in a table

libssh2-1 package basic information: