Update a value from a form to a related table field

I have a member DB in which members buy camp sessions and then use those sessions over time - member buys 10 sessions then comes in on future dates and “uses” the balance of sessions, each day using 1. They might also on a given day buy additional sessions as well as use the last one or so when the balance gets low.

So, I have a form that records the day of the “use” of a session > (Fields) Member-Date-Use-Add which must then update the balance of available sessions on the user record.

Table 1 - Camp member which has name, email and camp session balance.
Table 2 - Daily camp transaction using the session and or adding new sessions.

How do a get the daily form to update the balance in the table 1 with the sessions used and or added to the balance?
I can’t see a way to get the Submit button to have an action that updates the session balance.