“How can I” is the correct structure when you are asking a direct question. When asking a direct question you always change the order of the subject and the main verb
“…..how I can” could also be correct if the question is an indirect question. An indirect question has some additional words before “how I can” and is a way to be more polite in English.
Examples:
How can I open this door?
Would it be possible for you to tell me how I can open this door?
Notice that both examples have the same intention but the second example adds extra information.
How can I?
“How can I?” is correct when it comes at the beginning of the sentence.
Examples:
How can I find some meat for my taco?
How can I take the train?
How can I start a business?
How can I buy some beer if I have no ID?
How can I find a nice girl?
How can I work now with that baby crying?
How can I dance the Tango?
The above examples are in the normal structure of interrogative sentences