Pre-requisite: Make sure the account used is a service application administrator for the search service application that is being changed.
The new account which is going to replace the existing should be registered as a managed account already. Else need to register before changing in the properties page that is shown below.
1. Go to Central Administration.
2. Click on Application Management
3. Click Manage Service Applications
4. Select the service application that you want to change. Once you select, the Properties in the tab enables. Click it to open the Properties window
5. In the popup window, Select the account you want to change to.. here from SPS2013Farm to sp2013_search in the drop down in Search Service Account.6. In Administration pool for Search Admin Web Service, Check the account if it is also setup as sp2013_search and scroll down until the end to check Search Query Application Pool and then click ok to save the account.
7. Then click on the Search Service application again after the popup closes down. In the resulting Search Administration page, Click on the user account which is next to Default Content Access account and change it. When you click, it will ask for user name and password in a small popup. Enter the needed details and click on OK button to save.
[note: The below image was taken after the changes so the new account is already reflected.]