You go to the appropriate location page by searching for it, go to the checklist page in it, search for the taxa and edit from there.
Unfortunately it is a one at a time process, there is no quick way to do it if it is wrongly applied across a group of locales.
So for examples changes to Canada are made at (checklist page link is towards the bottom left)
https://www.inaturalist.org/places/canada
Alternatively you can go to the taxon page as you have done, click on the status tab and if it is listed there, click the ‘view’ link to go to the edit page. Please note this list only displays 100 entries, and can not be expanded or scrolled through, so in particular if it has been cascaded down to lots of smaller checklists, there is no guarantee your desired geography will be avaiable here, in which case option 1 above is your only choice.
To newly add a missing checklist entry, you basically follow the same steps as in step 1, but on the checklist page itself, rather than using the search, use the add function (I’m interpreting your request to mean how do I add species X to checklist y if it is missing)