How do I modify an existing a sheet in an Excel Workbook using Openxlsx package in R?

Try this:

wb <- loadWorkbook("Revenue.xlsx")
writeData(wb, sheet = "Data", Rev_4, colNames = F)
saveWorkbook(wb,"Revenue.xlsx",overwrite = T)

You need to load the complete workbook, then modify its data and then save it to disk. With writeData you can also specify the starting row and column. And you could also modify other sections before saving to disk.

I've found this package. It depends on openxlsx and helps to insert many sheets on a xlsx file. Maybe it makes easier:

Package documentation

library(xlsx2dfs)
# However, be careful, the function xlsx2dfs assumes
# that all sheets contain simple tables. If that is not the case,
# use the accepted answer!
dfs <- xlsx2dfs("Revenue.xlsx") # all sheets of file as list of dfs
dfs["Data"] <- Rev_4   # replace df of sheet "Data" by updated df Rev_4
dfs2xlsx(dfs, "Revenue.xlsx") # this overwrites the existing file! cave!