How to replace / subsitute using index / match?

[Gazsim]

Hi All,

I am looking to create a formula that can replace an existing number in a table with a new one without having to manually overtype it. I have used an INDEX / MATCH formula to locate the current amount or number in that particular month by supplier but can't go any further. I have attached a basic overview of this so in this example the idea would be for the number in cell K6 to be able to replace the existing number of £50 for Jon Smith in January for example. If I then say select BMP in February the formula would again have to replace this.

Hope this makes sense?

Many thanks,
Gazsim.