You need to update your programming skills in R programming regularly. It can only be possible by practicing every day. Write code on your own and validate your code with the solutions on the Internet. Consider time complexity and space complexity while writing R code. There are a lot of online competitions for R programming. Participate in those competitions to check where you stand in the leader board. In case of any queries, you can check R documentation or R community for solutions.
You can watch this video on R programming tutorial for a quick revision before practicing R: