We need more context to say for sure but I don't think either of your options are possible. I would normally interpret 'I'll have it by tomorrow' to mean 'I will receive it by tomorrow' e.g with 'it' meaning a physical item e.g. a package, a parcel, a ticket, a contract, a report and so on. Another possible interpretation for the phrase would be 'I will have the answer by tomorrow' i.e. the person does not need much more time to solve or answer a problem.

This sentence suggests that something that is not in your possession now will be in your possession by tomorrow. For example "have you got that report from head office yet"? "No, they promised me I will have it by tomorrow".