Help please

Help please

Hi - sorry do not know the terminology in zoho... but I have an Zoho app for my business  - reminders are important to my business, but two different programmers working on this have failed to come up with a solution. They both say design is the issue.
 
So instead of trying to fix what has been created - I am looking at a new  fix ,  how I used to solve this problem in Access.
 
Essentially if I invoiced an item eg: ProductID =234 ( Annual vaccination) then I would post this record as a date to the Annual vaccination field,  in the Pet Table.
 
I should be able to create a schedule to e-mail the client when the date is due from the Pet form/ table.
 
The question ??
 
What is the script to update a field from one form to another form ?
 
Thanks
 
Garry