Need a help to update a Sales Invoice Items

I have 4 fields in my sales invoice items and need to update, i already add 3 more field in table.

Fields are (Text Forms), (No. Of Colors), (QTY(Already in The System)),(Rate(Already In The System)) (Total(Already In the System))

How can i multiply all 4 and update the total of all

Please help me to update the form

Please check the following post to learn how to write a custom script to calcuate value based on the value in various other fields.