Global tempo change (percentage for all tempo marks in a score) ?

Hi,
I couldn't find the question nor the answer in the forum.
After spending quite some time creating tempo marks in a score, I'd like to slow down the complete piece, just as can be done in the Play Panel.
Is there a command to change all tempo values in a score by a given % ? Doing it one by one would be very time consuming en could even introduce errors.
Thanks.