In the web UI, click on the task icon, then click on “See more” in the command menu that pops up, then click on “Edit Runtime”.
You will be presented with a bunch of editable job configuration items (i.e., corresponding to settings under [runtime][TASK] in the workflow config).
Once submitted, the edited settings remain valid for any future job submissions by the target task, unless you re-edit to cancel the changes via the web UI.
You can also view, change, or cancel these settings using the cylc broadcast command, which is how this works under the hood in Cylc 8.