As Boris mentions, that is just Github trying to being helpful, and knowing that Jeremy’s repository is the parent and you have a branch with new changes, it is suggesting to create a PR. If you check back later (tomorrow perhaps?) you wont see those messages any more.
Updating the same PR is not implemented at the moment, but it is possible but will require a bit more work. This is how far I got with missing my bed time by a couple of hours last night! @pmario and I have been discussing what it would take to also be able to update a PR over in the other thread: Edit documentation in TW and use Github API to create PRs directly? - #14 by saqimtiaz
Personally I think it would be helpful to figure out this part of the user story and workflow first, creating a new PR and how and where the user starts with a copy of tw-com. Updating PRs can come in time. The UI needs work as well and that is all wikitext work.
Hopefully there is enough there at present to inspire others to contribute towards getting this usable by others.