Pentax K20D - Manual (Page 233)

232
Cam
e
ra
Se
tti
ngs
10
4
After entering the desired characters, press the { button.
The text is changed.
5
Press the 3 button twice.
The camera returns to the Capture or Playback mode.
You can reset the file name to the default setting by selecting [Reset File
Name]. (p.241)