A contribution rate changed this year. How do I update every line at once?

Edit the rate in the sidebar Variables tab, once. Every AddOn linked to that Variable recalculates, and so do the Section subtotals and the Total.

Prepared cards store their rate in a Variable from the first time you use them in a budget, and dropping the same card on another Cost Item reuses that Variable. The whole budget therefore starts on one rate.

Change the rate everywhere

Open the sidebar Variables tab. The panel is headed Variables in this Budget.
Find the Variable for your contribution and type the new value.
Check the Total. Every linked AddOn has already moved.

Link any AddOn to a Variable

Create the Variable if it does not exist yet: + in the Variables tab, Create new Variable, a name, and % Percentage as the type.
Expand the AddOn on the Cost Item and click its rate field, which reads Number or @.
Type @, open the Variables tab in the menu, and pick your Variable. Variables you create yourself appear under Preset.
Repeat on every AddOn that shares the rate.

When one line did not move

A rate field holds either a Variable or a typed number. A linked field shows the Variable as a grey chip with an X. If you type a number into a field, that line follows the number instead of the Variable, so a later rate change leaves it alone.

To bring the line back, click the rate field, type @, open the Variables tab, and pick the Variable again. The chip returns and the line follows the Variable from then on.

AddOns in Individual mode keep a rate on each row, and the Unified rate does not reach them. Link each row to the Variable the same way. Unified and Individual keep separate settings, so a rate set in one mode does not appear in the other.

Good to know

A Variable's type is fixed when you create it. Choose % Percentage for a rate, because a 123 Number Variable does not become a percentage later.
Deleting a Variable that lines still use keeps their figures: every place that uses it is replaced with its last value, and the confirmation states how many. Those lines then hold a typed number and no longer follow any Variable. Read the count in the confirmation before you confirm.
Each scenario keeps its own copy of the Variables, so a what-if rate never leaks back into the main budget. Change the rate inside a scenario to test it there.