If you need to reassign a user account to another sales rep for any reason (change of employee, replacement of a sales rep etc. . )

Go to Settings -> Users -> Manage Users

Select the user you wish to change and click the pencil/Edit icon.

Change the name and details and most importantly the email address of the user, this will be the new user's login.

The password will stay the same as that of the previous user.

To reset the password, select the user and click on the reset password link.

A confirmation screen will pop up showing the new password. The user will receive an email with the new password.

Alternatively, the new user can click Change Password when they login and provide the old password and change it themselves.

Make sure you assign the new user the appropriate Accounts via the Accounts file - using the user's Sales Rep ID. For instructions read this article:

How to Assign Accounts to Sales Reps 

Historical data and activities assigned to the old user transfer to the new user.

All historical data (orders, activities, targets) once associated with the old user remains associated with the new user even if you changed the Name, email and/or Sales Rep ID.

If you delete the user and create a brand new one, then the new sales rep will start from scratch, with no historical data.