I changed my subscription for my home internet, but I still pay the old amount

I am woth KPN for more than 6 years i made home internet contract last 6 years until now, since 2018 they charged me €41 to 43 every single month on my tbis year they asked me to renue the contract from online to give me good ofwr which is €35  a month for first 6 month but only 1 month the charged me 35 other months back to €43

Evwnt every weekend they notifing me they qorking on the Internet conection will stop for few hours but that is not fear , i will ask for some mony back, how you can take €43 every month from my bank account  i need at lest 2 for 4 payment back .

By they way i alredy spoke to customer service alredy know my stiuaition.

Kinds regard

Hemn