How to send field value of Child object to it's Parent object in Salesforce?

It depends on a few factors:
If it’s a master detail relationship and you want to roll up a number type field (number, currency, percent), you can create a roll up summary field on te parent.
Otherwise, create a field on the parent as needed and use this free package:
https://github.com/afawcett/declarative-lookup-rollup-summaries