How do I use VB to update a shared file

[memphid_head]

I need to know if anyone can help me with vb code. I have a shared excel file that many users will use. my form writes data in the next available row. But when the users save, a conflict error will ask to replace or keep data in the cell. I need some code that will update the file before it writes in the row incase another user has written to that row.
Please can you help me???!
Many thanks