Hello! Once you're in the edit mode in plaintext view, you can actually click on the particular text and start typing away to make your edits. Being plaintext, there aren't any fancy buttons or a text toolbar :)
I've included a link below which has some great tips on cleaning up your plaintext as well
In case some other folks are following along on this thread, I wanted to include steps of how to switch to plaintext view. Once you’re in your campaign, you’ll see in the upper right corner under the review and send button where it says normal view. You’ll want to click on this and you’ll then be prompted with a drop down menu.
You’ll want to click on Plaintext which will then open up the plaintext view. Please let me know if you have any questions.