You can set it globally with the variable org-deadline-warning-days. For example, to start showing all tasks 3 days before the deadline:
(setq org-deadline-warning-days 3)
If you want to set it individually, you can use the following construct in the timestamp itself:
DEADLINE: <2021-07-20 dom -3d>
Here, you can also use other modifiers, such as ...
Add (global-auto-revert-mode 1) to your init to automatically refresh files changed on disk.
As the doc states (C-h f global-auto-revert-mode):
If called from Lisp, toggle the mode if ARG is ‘toggle’. Enable the
mode if ARG is nil, omitted, or is a positive number. Disable the
mode if ARG is a negative number.