URGENT I need help with Microsoft Excel Chart

I would like to change data on sheet1 in chart1.

I put one graph (Chart1) in my form and I would like to modify chart1. The chart1 was build with data in sheet1. How can I change data on this sheet and please give me and exemple.

I try with: Worksheets("sheet1").cells(2, 3).value = x but the method is not good because I received all the time this error "Method or data member not found".

notes: The sheet1 was in the objet MS Excel chart and is name is sheet1.

Sorry for my english.
Thanks
Redg