How to change value of a column in one table after a value changes from another

[Tiger1]

I have two tables:

Table A

Table B

I need Table B column [Quantity] to change through automation after Table A column [Quantity] updates.

[Hussein_Osseily]

You can create an action that would be triggered when a form is submitted.

Create the action to lookup the new quantity from table A 

Go to form view of table A 

Behavior 

Select the created lookup action .

Now everytime anything is changed in this row will trigger the action to change quantity in table B 

You can take it further and a change counter column for the quantity column and create a bot to do this.

Option1 should work for u too