Learn Russian: Ты работаешь в офисе пять?

Just like in English, Russian uses cardinal numbers (one, two, three, etc.) right after a noun to act as a label or room number. So офисе пять simply means 'office number five'.

Since we are saying 'in' the office, shouldn't пять also change its ending?

Not in this sentence! Because пять is acting as a label or title for this specific office, it stays in its basic dictionary form. The noun офисе takes the ending for the location, but the number tag doesn't.

Why does работаешь end in -ешь?

This is the present tense ending for ты (you, informal). When you want to say 'you do' something for most regular verbs, you drop the dictionary ending and add -ешь. Here, работать (to work) becomes работаешь (you work).

Why does офис have an -е at the end here?

Because it follows the preposition в (in) to show a location. When answering the question 'where?', the noun must be in the Prepositional case. For masculine nouns ending in a consonant like офис, you add an -е, making it в офисе.