Here is one way to use with_tz to change values in a data frame.
library(lubridate)
DF <- data.frame(DateTime = seq.POSIXt(from = ymd_hms('2021-01-11 08:00:00'),
to = ymd_hms('2021-01-11 11:00:00'), by = "hour"),
Value = 1:4)
DF
DateTime Value
1 2021-01-11 08:00:00 1
2 2021-01-11 09:00:00 2
3 2021-01-11 10:00:00 3
4 2021-01-11 11:00:00 4
DF$DateTime <- with_tz(DF$DateTime, "US/Eastern")
DF
DateTime Value
1 2021-01-11 03:00:00 1
2 2021-01-11 04:00:00 2
3 2021-01-11 05:00:00 3
4 2021-01-11 06:00:00 4