This method allows you to simply change the machine type of your VM. You might want to increase the CPU count or RAM or change the CPU category. Note that from 01.08.2021 (first of august) on you have to use the e2-micro (2 vCPUs, 1 GB memory) type to apply to the always free tier.
First of all you have to navigate to VM Instances. There you need to click on the free dots besides the Connect Button and select Stop. When the instance has been successfully stopped you need to click on the the instance name to open the instance details. There you need to select Edit at the top. Then scroll down to where you can select the instance type and change it to the one you want. Be aware that this option is only available when the instance has been stopped. Then you just need to start the instance again, the new CPUs and RAM will be used and charged.