Is there a way to select the executor from the UI?

Hi @Diego Alonso Roque Montoya - no, that's not possible at this time

Is it exposed in anywhere in the prefect.Client?

No - the executor is configured in your code. The UI does, however, allow you to configure your run config for ad-hoc runs through the UI.

I'm using a Dask cluster that I spin up just before te flow. Is there a way of setting the address of the cluster just before running, instead of before registering?

Hm no, however you could use a ResourceManagerTask to configure this as part of your flow and parameterize your execution that way. Note that this will shift the setup and teardown of your cluster into your resource manager's setup and cleanup tasks (so you'll need to do these manually instead of having Prefect do that for you).