How to merge specific columns with its next column without hardcoding in R programming

Question

How to merge column names that are "X" with its next column without hardcode in R programming

X should be merged to Day.7
X.1 should be merged into Day.8
X.2 and X.3 should be merged into Day.9

Code:

library(data.table)
library(tabulizer)
pdf_file <- "new.pdf"

out2 <- extract_tables(pdf_file, pages =c(89), output = "data.frame")
out2<-as.data.table(out2)

colnames=colnames(out2)

output:

"G" "X"     "Day.7" "X.1"   "Day.8" "X.2"   "Day.9" "X.3"  

Expected Output:

"G" "Day.7"     "Day.7" "Day.8"   "Day.8" "Day.9"   "Day.9" "Day.9" 

PDF DATA is added here: