How to add up values in Dataframe in R

Question

I have a sales dataset in R for one country. It contains 23 products and shows for each the monthly turnover, units sold, etc. I'm trying to create a chart that shows that total units sold by each month grouped together, how can I add up the corresponding months together now? So that instead of having 276 rows, my new data frame has 12 rows for each month showing the values. Dataset example