H1-H6 (hx) is easy for you to fix yourself. Depending on the type of page you are using would change how you do that change, but it’s simple to do.
Did they send you the actual HTML meta tags lines of code? If they did you can simply place them as is in the head section of the page.
Without seeing what they sent you it’s hard to say for sure.
You said that this is for a client, so it’s probably a good idea that you learn how to do this kind of stuff yourself. It would really cut into your profits if you need to subcontract simple things like this. With a sample of what they sent you you can get good help here.
Also, you might want to understand and make sure your client knows what they can expect from what they have been sent. The h1-h6’s can and probably will impact the SERP ranking. Google and Bing both have made it very clear that the metadata (stuff not seen by the end-user) will have no impact on search result rankings.
It might (less and less) impact what gets displayed on the SERP(search engine results page). Google in particular is using their own description based on the content and matching what’s being searched for.