From the MTA:
From 11:45 p.m. Friday, April 20, to 6 a.m. Saturday, April 21, R trains will be suspended in both directions between Whitehall Street and 59 Street due to switch maintenance. R shuttle service will run between Bay Ridge-95th Street and 59 Street. Transfer between R and N trains at 59 Street, and between N and Q trains at Atlantic Avenue-Barclays Center.
From 6 a.m. to 11:59 a.m. Saturday, April 21, Manhattan-bound R trains will run express from 36th Street to Atlantic Avenue-Barclays Center and skip DeKalb Avenue due to switch maintenance. Use a Coney Island-Stillwell Avenue-bound N local or Bay Ridge-bound R to complete your trip.
From 6 a.m. to 11:59 a.m. Saturday, April 21, Manhattan-bound R trains are rerouted via the Q (Manhattan Bridge) from Atlantic Avenue-Barclays Center to Canal Street due to switch maintenance. Use the Q instead which runs via the R (Montague Tunnel) during this time. Transfer at Atlantic Avenue-Barclays Center or Canal Street.