Did you already create the features you want in Back Office?
After that
I would recomend to you download and install this script:
https://www.prestashop.com/forums/topic/185401-free-script-prestools-suite-mass-edit-order-edit-and-much-more/
(if you want to pay the pro version of this script you will be able to change the features otherwise you will need keep reading)
After install you will need choose the fields name, reference and categories and save a csv file with those.
Now You need install:
https://www.prestashop.com/forums/topic/37900-free-module-products-export-module-v253-updated-23032016/?hl=%2Bexport+%2Bmodule
export all your products
Now you need to open those 2 csv files
And look for the field features and replace all the final ":1" for ":0"
"Feature (Name:Value:Position:Customized)"
"Altura - Produto artesanal - Pode variar entre :Almofada Peq 28 e 32 cm. Almofada Grd 38 e 42 cm:1:1,Largura - Produto Artesanal - Pode variar entre:Almofada Peq 28 e 32 cm. Almofada Grd 38 e 42 cm:2:1,Peso - Produto Artesanal - Pode variar entre:Almofada Peq 280 e 320g Almofada Grd 580 e 620g:3:1,Tecidos:Veludo:4:1,Estilo:Jogos:5:1,Composição:Plumante 100% silicone:6:1,Lavável:Sim:7:1"
See the red :1 for example above
Now you can copy the feature field to the csv file you made with the script. (pay attencion not mix the products)
Now you can save this final csv file and upload to your store again.
If you upload only the features without the category field all your products will lose your default category and will become HOME.
The export module is good but doesn't save the default category like script does.