A quote status can be changed to 'Accepted', but this will not turn it into an Invoice.
On the Web Portal, go to CRM > Quotes > Edit pen for the quote > select the Status of accepted from the dropdown > save.
On the App, go to Accounts > Quotes > open the Quote > select Edit at the bottom > select the Status of accepted from the dropdown, just under Notes > Save.
A quote can be converted to an Invoice once the quote has been saved.
Your clients can also Accept their Quotes when you send them, if you have QuoteAccept enabled.