How to grant full access of an account to a user inside the organization?

Step 1: Sign into Office 365 admin portal via https://portal.office.com

Step 2: Click on Admin from the left pane and navigate to Exchange admin center.

Step 3: Click on the Recipients and switch to Mailboxes tab.

Step 4: Click on the mailbox you would like to grant access and click on Edit icon on the top-left.

Step 5: Switch to Mailbox Delegation and scroll down to Full Access section.

Step 6: Click on the Add icon and select an user from the list and then click OK.