You can push a domain to another Namecheap account in the following way:

• Sign in to your Namecheap account
• Mouse over your username at the top of the page
• Click on Manage Domains
• Click on the domain name you would like to push. If you need to push several domains, tick check boxes near the domains and click Edit Selected below
• Click on Push Domain To User on the left-side navigation bar

You will need to enter your account password for authorization and the destination username the domain will be pushed to.

Please note that destination account user has an option to enable additional security feature. If it’s enabled, you will be required to enter authorization code during the push.

NOTE: Do not mix up Push authorization code with Transfer authorization code (EPP). The latter is needed only for domains transfer between registrar, while the Push authorization code is needed when transferring domains between Namecheap accounts

How to get the authorization code?
Please contact the destination user to find out what the code is. The destination user should follow the steps below to get the code:

– mouse over Namecheap username on the black bar at the top of the page;
– click on Manage Profile;
– click on Push Settings tab in the left-side navigation menu. If the option is disabled (i.e. Allow All is selected), please leave the code field blank during the push.

If you are the owner of both original and destination accounts, copy the code from the destination account by following the steps above, log out and sign in to the original account.

The push is free of charge, and the domain you push is not renewed.

Also you should select whether or not you wish to change the contact information for the domain to the one of the new user. If you select to change it, the domain’s contact information will be set based on the information available in the destination user’s profile, otherwise your contact details will be left intact.

Please mind that the push is final and we will not be able to reverse it. If you need the domain back you will have to contact the new owner of the domain.

You can check the following video tutorial on the domain push:

Source: https://www.namecheap.com/support/knowledgebase/article.aspx/325/46/how-can-i-push-a-domain-from-one-account-to-another