Hi @HABIB1 
Yes, you can do that.
But also keep in mind that you can currently receive a cancellation SMS after 15 months without charging.
Therefore, pay attention and top up your credit every 15 months.
Means the number remains even if there is no credit.
However, the prepaid tariff cannot be booked and remain active without credit.

Hallo, 

No problem for me to top up every 15 months. So please how can I do to change the plan? I can't do this on the app. I am leaving Germany on Monday. I  called today Telekom online service and they told me if they want to terminate the plan, I will loose my number. The only way is to keep paying monthly even if I am outside Germany. I am bit lost.

You can also top up using GSM code.
And as long as you don't report a move, nothing will happen when you cancel with Prepiad.
But that could be less next year.

How to top up your prepaid credit:
If you have purchased a receipt for topping up, the top-up code is visibly printed on it. With a top-up card you have to scratch off the code printed there. To recharge, enter *101*Recharge Code# and press "Pick Up". The new credit will then be available on your prepaid card.
You can also top up new credit by calling the speed dial 2000, which is free within Germany. To do this, simply follow the menu.

https://www.telekom.de/mein-kundencenter/guthaben-aufladen 

Sorry but my question was not clear. How can I do to stop the monthly debit of 4.95 euros when I leave Germany? I can top up with 15 euros via the app, but the problem that  I need to pay 4.95 euros every month even if I am not in Germany. 

Actually the only way is to not top up your card and keep your balance at zero if you don't use the card. If there's not enough money the tariff won't be renewed, once you top up your mobile account the tariff gets active again. The German Telecom does not offer a tariff without basic fee you can switch to.

If you want a tariff without monthly fee you either need to buy a new card in the designated tariff ("MagentaMobil Prepaid Basic") which comes along with a new number or you need to transfer your number to another provider which offers something more suitable for your needs (there are other brands of German Telecom such as congstar you can look at).

Ok if there is not enough credit, the tariff won't be renewed. But in this case for how long time can I keep my number active  without top up? Is it every 15 months?

Ok I understand now. Is there any minimum tariff to top up? For example can I top up with just 5 euros from any Telekom Shop (and not via the app) to renew the 1 Gb?

Also If I am going to any EU country and not to Germany, is there any way to top up (with just 5 euros for example) from my country without using the app?

If you can catch a 5€ voucher it would be sufficient. Usually you get only 15€ and up-

→Mataimaki←

You can also top up using GSM code.

If you have a code you can dial *101*code# to top up. This is without extra cost matters not if you're in Germany or abroad.

There are online shops available selling such codes.

If you have a German bank account you can top up through online banking.

General overview with all options: https://www.telekom.de/mein-kundencenter/guthaben-aufladen

Ok I see now. How do I get the voucher for 5 euros? Is it just from any Telekom Shop? If I am in a EU country, can I get such vouchers?

Seems they’re online available as well, I got some results searching for it.