Create an Update on a Form (1 Viewer)

I am designing a form to assist the treasurer of our club to record bank deposits.
We have a calendar of scheduled events for the year. Every day has a fixed time for the events (from 1 to 4). Money is collected for each event and kept separately. Deposits are made once a week
My main table is “BankDeposit” with a multi-field key of DepositDate+DepositNumber.
The child table is “ActualEvent” with a multi-field key of EventDate+EventTime with a foreign key of DepositDate+DepositNumber to “BankDeposit”. It contains detail of the proceeds and several different discounts with a calculated net for each event.

I have a main form, Bank Deposit, with a sub form, Actual Event.
My idea is to have the user enter the Deposit Date & Deposit Number and then the range of dates/times of the events income included in the deposit.
Then I would like to be able to update the ActualEvent table with the Deposit Date and Number. Then I want to refresh the sub form so the detail of each event can be filled in. Detail of the proceeds for each day can then be entered and a total deposit calculated.

I am unable to create actions to accomplish the update and refresh the sub form. Please help.

I started reading your question and then I got to the bit where you mention multi key Fields. I'm not 100% sure what you mean by this, however if you mean that the unique ID of the table is controlled by two Fields, then I strongly recommend you just have the normal autonumber field. It's just so much easier.

With regard to:-
>>> and then the range of dates/times of the events <<<

Do you mean you want to automatically fill the subform with records that relate to that statement?

I think I'm trying to put too much info in my request.
1. In my main form I want to enter:
UNIQUE_ID
Date Range
2. Cause an action to update all records in the Date Range in another table
with the UNIQUE_ID from 1.
3. Cause an action to "refresh" the subform with the updated records from 2.

Here are the specifics (might not clarify):

My form has a parent table, Bank Deposit with a unique ID and a child table in a subform.
The child table contains a row for each pre-scheduled event for the next year. It eventually will have detail of proceeds and different discounts for each event. However the rows in the event table are not tied to a parent Bank Deposit record yet, so those event records don't show up in the subform.

Here's an example of what I see happening:

User enters into the main form:
Deposit date: 6/15/2017
Date range of events: 6/1/2017 thru 6/14/2017 (20 separate events)
Press some kind of action button(?) to update the 20 separate events in the
Child table with the Deposit ID for 6/15/2017.
Press some kind of action button(?) to refresh the subform so all of the
child rows appear in the subform since they now have a parent.

The user will now fill in all of the details for the proceeds of each of the 20 events. The subform will have an aggregate expression for the total proceeds that will be displayed on the main form. That gives the user the total amount of the deposit so she can fill out the deposit slip for the bank.

I hope this question is a bit clearer.