The easiest way is to use the software as a business or individual, provide feedback and report any bugs or issues you or your customers encounter.
Translating BTCPay, helping us with documentation, making videos and spreading the word are ways in which you can help, even if you're not a developer or tech-savvy.
We need more VPS hosting providers to implement 1-Click BTCPay deployment and make the installation of software more accessible, decentralized and widely-available.
We are actively looking for developers who can take on and resolve GitHub issues and help with development. If you would like to help us, but need some guidance, #dev channel on Mattermost is the place to ask questions.