Learn German: Ich hoffe, dass du das Update herunterladen kannst.

In German, you must always use a comma to separate the main clause (Ich hoffe) from a subordinate clause (the part starting with dass). Even though we often don't use a comma before "that" in English, it is a strict punctuation rule in German.

Why are both verbs (herunterladen and kannst) at the end, and why is kannst last?

The word dass (that) is a subordinating conjunction, which means it kicks the conjugated verb to the very end of the sentence. Here, the conjugated verb is the modal verb kannst. The main action verb, herunterladen, goes right before it and stays in its infinitive (dictionary) form.

I thought herunterladen (to download) was a separable verb. Why isn't it split up here?

It is a separable verb! However, because you are using the modal verb kannst (can), the main verb herunterladen is forced into its full infinitive form, so the prefix stays attached. You only split a separable verb when it is the active, conjugated verb in the clause.

Update is an English word. How does it get the neuter gender (das)?

When English words are adopted into German, they usually take the gender of the closest German equivalent word. Here, it likely takes the neuter gender from das Programm (the program) or das Aktualisieren (the updating). As a general trend, many English tech loanwords become neuter in German.