How do I replace a shower drain unit in a fiberglass shower stall?

How do I replace a shower drain unit in a fiberglass shower stall?

First, Why do you want to replace it? Some plastic and fiberglass has the drain molded into the base. If that's the case you can't replace them. If you have a metal (lead and oakum poured joint)(see image) or a plastic drain, (see image) ( cemented in) let me know. Back to you, Tom

Tom said it... Why ?

You can replace Strainer only. It covers entire drain inlet so if you put in new one it will look like entire drain was replaced.

Anyway, if you really want to replace entire drain - than I hope the shower is on upper floor or you have raised foundation. If positive, cut ceiling / floor, unscrew large drain nut, remove drain, install new one and reinstall the nut.