To change what Approval Scheme is attached to an Agreement, go to Agreements, locate and click on your Agreement so you see the below page layout:
Under the Approval section, click on the cog on the right:
You will then see a list of the available Approval Schemes:
The tick will indicate the current Approval Scheme attached too this Agreement. By clicking on the button you will replace the existing Approval with the newly selected one:
Once selected, click the Done button and it will take you back to the Agreement page and the Approval section will now show its new Scheme