I got a card which is not possible to install applets on it! even GP Verify is not executable! I asked the vendor about this and they told me to change ATR to get it working.
How Can I change card ATR without installing any applets?
With an Applet you can normally only change the ATR historical bytes. This is only when the Applet is the implicit or default selected applet. In other words, the applet that initially receives the APDU's before an other Applet gets selected can set the historical bytes.
The communication parameters may also be configurable, but this is usually accomplished by a vendor specific method. For instance, for NXP you can generally select a "root applet" and configure the chip and set the ATR. However, to do this you need to obtain the way to do this from NXP.
Normally you should try and contact your vendor first though. To be honest, I think that answer of the vendor is not detailed enough. Ask them for an example how to do this.