changing values from one entity into another

So I'm in the account form and I have a sub-grid in here for meetings. I have added custom button to this sub-grid so it must imitate the same behavior as the 'Mark Complete' of the meeting does. There is no change that I can trigger that button, so I have to write code.

My question is: 

How can I change the values of some fields for the item of Meeting entity(ofc I know the ID of the meeting I'm operating on) from the Account form ?!

How can I update the field values with making any kind of web request ?

Thanks.