Can I convert ERC20 to ETH?

I'm quite new to the cryptocurrency world and I've been hearing a lot about ERC20 tokens and ETH. I'm wondering if it's possible to convert ERC20 tokens to ETH? I've seen some exchanges offering this service but I'm not sure if it's safe or how the process works. Could you please explain to me how I can convert ERC20 to ETH securely? I'd also like to know if there are any fees involved in the conversion and if there's a specific time frame in which I should do it. Your guidance would be greatly appreciated!