since I have the free version the survey administration time is limited to 3 months and than I can extend 1 month
That is correct. If we did not introduce this limitation, everybody would enter 1.1.1900 to 31.12.3000 in their projects and we never knew which are actually online ;)
do I have to write like from today 29/04/25 to 29/07/25 right?
If that is a reasonable period for you to collect data, that would be possible, yes.
And in case I can extend 1 month the period
Close to the end on July, let's say 25.07.2025 (29.07. is also fine, however be sure not to forget) you can enter the then-date plus 1 month, for example 25.08.2025. You can repeat that for up to two years.
It's not a tragedy if you miss the last day of the survey administration period, however, the survey will go offline for a day or two, if you forget. If you're over multiple days, you may also have to set a new start date. This won't inflict your data, but the return graph will not display the whole period then ... easier just to extend the duration in time.
can I change like questions, texts or sections of my project right?
Sure. And as you read the manual, you already know not to delete items or questions, because you'll not be able to download the data colelcted for them earlier, then.