[Geoff Bacon]

Long press and then release one of the songs; a check box will appear next to it.
A floating toolbar will appear near bottom right of screen - tap the open square box to select all (the other one clears the selections)
Note: this is for an android tablet, I don't know if you are using windows or if it is the same on windows.

Another alternative, if you have an android tablet and a windows pc, is to use the Companion program.
This allows you to use common window actions to select songs e.g. CtrlA to select all songs or, for a range,  select the first song, scroll down until last song is visible, hold the shift key down and select the last song.
I quite often use the Companion for manipulating the files.

Another tip is to make a backup of the database before making major changes (you can do this from MSP but I use the Companion File option)

Geoff