HI @siusun,
Are you using RStudio Server or RStudio from your desktop? You might want to move this question to the #rstudio-ide category, to make it easier to find for those who know more about this.
Here's a good, quick, pictorial of how to do that from the meta-Discourse site:
You might also want to add some more information about your system (what OS you're using, etc.)