Hi @RebeccaIsolde ,
Could you please tell me in which scenario you are looking:
1. If you want to insert a new iframe into the experiences, You can create an activity using Visual Experience Composer, and insert iframe content.
2. If you want to change the content inside an iframe, the iframe should be under the same domain, and in the custom code section, you need to insert code to change the content.
Note: Changing content in third party domain is not possible.