Hi @wandyoroh
Here's the [https://download.lenovo.com/consumer/mobiles_pub/s340_hmm_201902.pdf|hardware maintenance manual] for the laptop.
Go to p.44 (p.49 of 84 pdf) to view the procedure to remove/replace the fan. This will allow you to gently clean the fan outside of the laptop case without getting more dust etc into the laptop whilst cleaning.
Follow up by removing the heatsink as shown on p.48 (p.53 of 84) so that you can remove/refresh the thermal paste on the CPU. Here's the ifixit [guide|744] guide that will help with this.