How you do that depends on the species you are working with. Let's assume you have human data.
> library(org.Hs.eg.db) > select(org.Hs.eg.db, "U43746", "ENTREZID","ACCNUM") 'select()' returned 1:1 mapping between keys and columns ACCNUM ENTREZID 1 U43746 675
There are many orgDb packages that you can install, and even more that you can get on the AnnotationHub. I would recommend reading the vignettes for the AnnotationDbi and AnnotationHub packages to get yourself acquainted.