Hi deepthi.lite,
This is a premium features. Upgrading to premium version you can change the value of excerpt from customizer. If you like to change the code then goto theme folder -> inc -> functions -> functions.php on line no 148 change value 50 to any value you want.
We recommended you not to change the code. If you change the code then while updating to new version all your customized code will be lost. So better make a child theme so that your customized code will not be lost.
Thank you