The client cut a shoot day. How do I update everything that depends on it?

Shoot days drop from five to four two days before the bid is due, and every crew line, rental and per diem depends on that number. Put the day count in a Variable and point the lines at it, and one edit recalculates every field that references it, along with the Section totals and the AddOns sitting on those lines.

Set the day count up once

Open the budget and click the Variables tab in the sidebar.
Under Variables in this Budget, click the + button. Its tooltip reads Add new Variable. On a budget with no Variables yet, click Add Variable in the empty state instead.
In Create new Variable, enter a Name such as Shoot Days, leave Type on 123 Number, enter the Value, and click Create Variable.
Go to a line that depends on the day count. In its Days field, type @. The reference menu opens on its Variables tab.
Click your Variable. The field is replaced by a chip showing the resolved value.
Repeat for every dependent line: crew, rentals, per diems, vehicles.

When the number changes

Open the Variables tab, type the new figure into the value box beside the Variable, and click away or press Tab. Every field pointing at it moves, and the sidebar Total follows.

Good to know

Variables you create are listed in the @ menu under the heading Preset.
Pointing the dependent fields at the Variable is a one-time pass through the budget, line by line. After it, every change to the day count is a single edit.
The Name is unique within the budget and the Value is a plain number, digits with an optional decimal point. Create Variable becomes available as soon as both are right.
A Variable belongs to one budget, and each scenario holds its own copy, which is what keeps scenarios independent. Running scenarios? Set the new day count in each scenario column you want to change. I want to try a different rate in one option without touching the others covers how that works.
When you delete a Variable that fields still use, each of those fields takes the Variable's last value as a typed number. The confirmation names how many fields change, and the deletion is final.