Canvas like many materials will relax over short periods of time.
As said set it up stake it tight. Put blocks uder the legs and keep pulling tight. Wet it.
Pay close attention to the zippered areas ,door,windows, not to over stress them.
It might take some time, but you have perfect weather for it.
Once you get it back where you want it. Make. Sure it drys real good.
Sod flaps will hold water and moisture so when you are ready to take it down , pull the bottom stakes and lift the sod flaps up to dry a couple hours before take down.