The Edit Menu

Edit->Undo (Ctrl-Z)

Undoes the last change

Edit->Redo (Ctrl-Shift-Z)

Redoes the last undone change

Edit->Select All (Ctrl-A)

Selects all clusters

Edit->Select All except 0 and 1 (Ctrl-Shift-A)

Selects all clusters except artefacts (cluster 0) and noise (cluster 1)