Hi, and welcome!
Please see the FAQ: What's a reproducible example (`reprex`) and how do I do one? Using a reprex, complete with representative data will attract quicker and more answers.
In this case it looks like you've named your data frame df, which is also the name of an R function, which is what appears in the post.
The easiest way to remove a duplicated column, say column_dupe is
my_df %>% select(-column_dupe) -> my_df
For columns 3 and 4 it's not clear what is duplicated. Do you have a row named waves?
If so, you may want to consider reorganizing your data frame to a tidy format, with variables, such as wave represented as columns and observations as rows.
In general, it's not a good idea to hand edit a data frame. If you do, meticulous notes are necessary to have any hope of recreating results.
To change just the single value 8 to a 12 can be done with subsetting.
Data frames are indexed row first, column second. So, for example, if the value 8 appears in row 15, column 2, the value can be replaced by
my_df[15,2] <- 12
Preferable, however, running down the error in the source.