Trying to change timezone and format on a date time column in R

Question

I have downloaded some hourly Snotel values, data frame is three columns: dateTime, SWE, and Snow Depth. The current format from the csv for dateTime is "YYYY-MM-SS HH:MM" and the timezone is MST. I would like to change the timezone to UTC to match my own data and slightly alter the format to be "YYYY_MM_SS HH:MM:SS". I have been flailing trying to use as.POSIXct, parse_date_time, as_datetime, and others. Is there a function I am missing that could do this?

Thanks