It certainly is recommended to amend since including HSA contributions more than likely will have a favorable impact on your return.
Here is information on amending a return. Note the amended form (1040X) is not available yet, but when it is the instructions below will be updated for 2018. You also will need to remember to amend your state return, if impacted. Also, the amended returns must be printed and mailed, as they are not available for e-file.
https://ttlc.intuit.com/replies/3318732
https://ttlc.intuit.com/replies/3288565
