How to change master password of Koha ?

Koha master login username and password is same as MySQL database (e.g. koha_library), where data stored.

First change the password of koha_library database.
Then put new password in /etc/koha/sites/library/koha-conf.xml

Apply following commands in Applications > Accessories > Terminal

sudo su

[enter the Linux Root password]

mysql -uroot -p  

[enter the MySQL Root password]

use mysql;
SET PASSWORD FOR 'koha_library'@'localhost' = PASSWORD('koha123');
flush privileges;
quit;

Need to change new password in Koha configuration file.

Open the configuration file using any text editor. Use any text editor, gedit/leafpad. Leafpad is not available, install;

apt-get install leafpad

sudo leafpad /etc/koha/sites/library/koha-conf.xml

The file will open. Scroll to bottom part of the file and find the line for password.

You can find old password. Place your new password.
Save [Ctrl+O] and close [Ctrl+x] the file. Login Koha using new password.
Reference