Here are instructions for changing the SIM card in a Huawei B535 router:
1. Turn off your router by unplugging the power cable or switching off at the mains.
2. Remove the old SIM card – in order to release the SIM card from the slot you need to press the SIM card in slightly, you will hear a clicking sound
3. Insert the new SIM card – press it in and you will hear a clicking sound again.
4. Plug in the power cable again or switch on at the mains.
5. Wait a few moments for the router to boot up.