在R中的数据框中查找，替换和丢弃值

I'm trying to figure out how to find, replace and discard values in dataframe in R. I'm using the built in Loblolly data set in R. I want to replace the height data recorded during the growth of seed 311 with NAs; and then discard all data from three-year-old trees.I'm trying to figure out how to find, replace a