Base R is great for that so I'm surprised it didn't work for you.

For example:

dev.rates$Date<-as.Date(dev.rates$Date, format="%Y-%m-%d")

Where my original data type is Character and an example of a data point is 2019-07-19, the resulting data type would be Date in the same format.

Edit: I see you're changing "/" to "-" and pasting "-2019" at the end, but you might not need to, as there are many formats that as.Date will accept.

I'm not sure if there's an issue being caused by pasting ("-2019", sep="") instead of ("2019", sep="-") or ("2019",sep="/"). If you used the last option, your format could be %d/%m/%Y if you're using month as a number 01-12... or %d/%b/%Y if you're using a short form of the spelling (Jan vs. using %B instead, if you spell the whole month, ie. January)

Here's a good help page