If you just want a Don for one month then buy it and the Guido auto renewal will not happen for that month. If you want to change to Don from now on then go to the auto renewal section of your My 48 account page and change the selected membership the from Guido to Don. Remember that EU roaming will only work on Guido so make sure you get Guido in time before traveling if applicable.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.