combine files tascam dr-05

[MikeShriner .jr]

I have just recently bought a tascam dr-05 recorder, ive got all of the functions down for the most part. but for some reason i cannot combine a file (or undo) that i had accidentally split into three files. I feel really upset that i have not yet figured this out by myself yet with just the owners book and youtube tutorials.
is there anybody that can help my solve this small yet minor situation?