If you want to change any G/L account, or you are configuring it for the first time, and you are not allowed to choose different account types (e.g. Liability accounts, Revenue accounts, etc), please check if this is allowed within your SAP settings.

Go to SAP- Administration- System Initialization- Company Details. 

Basic Initialization tab. 

The option "Allow Selection of Any Account Type for Revenue Accounts" must be checked as shown in the image below.