Hello Priti,
If you want business days to add, then use schedules. Refer to this article where i have shown how to add business days.
https://community.servicenow.com/community?id=community_article&sys_id=df6464c9db1ed4909e691ea668961...
If you just wnat to add 5 days then in teh business rule
Before update
var startDate = new GlideDateTime(current.planned_start); //check field names
startDate.addDays(5);
current.planned_end=startDate;
Mark the comment as a correct answer and helpful if this helps to solve the problem.
