Can we combine two invoices together?

Summary:

I have an invoice which comes in at the start of the year and is split over 10 months (lets say £100 total). Another invoice will come through mid way through the year which will be a reinstatement of the previous invoice (the amount is now £120 total). I have already paid off 5 months worth of the previous invoice (£50) but now I would like to group my updated invoice with my original invoice so the remaining 5 months is paid off with the updated invoice amount (£70).

Is there a way I can combine two invoices together to either:

a) use the original invoice and add the difference of £20 using the updated invoice?