Return and replacement of charger adapter

in many countries you also have 14 days full return refund no questions asked on online purchase

though you may have to pay for shipping the product back

if you bought it online with apple then you can try asking their official support channels

 Get Support (apple.com)

And or

 Contact - Official Apple Support

And or

Contact - How to Contact Us - Apple

Contact the store that you purchased the 20W adapter from and ask them if it is possible.

Hi, I have purchased it online!

Contact the seller from which you purchased the adapter to ask about their return policies.