In Salesforce Professional edition and above, you are allowed to rename system object label. Below are the steps involved to change Account object label to Customer:
1) Go to Setup > App Setup > Customize > Tab Names and Labels > Rename Tabs and Labels. Click Edit for Account.
2) Rename it to Customer
3) Once you click the Next button, you will reach a screen that allows you edit the Field Label.
Usually, Salesforce will populate this automatically using the new Label name. Once you have completed this step, you are done. You can also use it to rename your Custom Object tab name and label. However, even though that you have changed the Label name, the API name will still remain unchanged. For example: Even if you have renamed the Account to Customer, you will see Account__c in the API.