Updating from 3.9.x to 4.1.x

Hi, I have the 3.9 version installed, but now our client needs to activate the Stripe payment method, so we tried to update to the new version, but all the design crashed, so we reverted to the previous version. We see that in the new version there is no Page Builder.

How can we upgrade this theme without losing all the customizations and widgets/positions functionallities?

Or second way: can we only upload some files to add the Stripe payment method into the v3.9?

Thanks