Customize Ultimate POS (fosters) php program

I have hosted the php script on my server and it is the company script.
I wanted to make some customization to the project on hourly or full project basis.
Some changes are:-
1. POS SCREEN Discount system & preview
2. Adding products option( removal of extra stuff)
3. Bill layout full
4. Product tax slab
5.small changes

Demo link:https://ultimatefosters.com/ultimatepos-demo/?utm_source=codecanyon&utm_medium=description&utm_campaign=ultimatepos