Breaking mirror and recreating by logging into the desired account will be the easiest way. But we can follow the below steps to fix the account without breaking.
1. Query to find the current endpoint owner of the instance and take a note.
SELECT [PrincipalName] = sp.name, [PrincipalId] = sp.principal_id, me.* FROM sys.database_mirroring_endpoints me with(nolock)
inner join sys.server_principals sp with(nolock) on me.principal_id = sp.principal_id
2. New endpoint not exists then we need to create as login and need to be granted the following authorization.
GRANT CONNECT ON ENDPOINT::Mirroring TO [Domain\LOGINNAME];
ALTER AUTHORIZATION ON ENDPOINT::Mirroring TO [Domain\LOGINNAME];
3. Verify the current owner of endpoint using step 1.