Change whole project tempo when it has multiple tempo events

I have a project using the tempo track. I have set tempo events so that the metronome matches the tempo changes of imported audio.
Now I want to slow down everything, so audio and metronome(i.e. bars/beats) stay in sync . The only way I can find is to edit every single tempo event. Is there a way of changing all tempo events at the same time or some other way of acheiving this.

Select all tempo events and drag up or down…?

Looked for that. Can’t see how to select all tempo events.

Lasso select, or [strg]+[A]…?!

I can draw a lasso round them all and select them all but cannot see how to operate on the group. Any attempt to drag e.g. using Ctrl/Shift/Alt plus mouse button, just draws a selection square or creates a new event.
Windows btw.

Solved it!. If you open the info line it displays the Start, Value and Type. The Value field shows the Current Tempo just like the Toolbar. If a single event is selected the info line fields are shown in white. If multiple events are selected the values are shown in orange.
Double clicking on the Value field allows it to be edited. Select multiple events and then edit the Value. In this case ALL time events move up relatively.
So that was simple then. Undocumented (and I spent some time searching) Documentation only ever refers to changing single events.