As far as DIY bike replacements go, changing out a bike seat is pretty simple. You simply loosen up the nut keeping the seat on and pull it out for a more comfortable model. Unfortunately if it's been years since your seat has been removed or if you've never taken the bike out of storage for the better part of a decade the seat post might give you some issues when you try to replace it. A stuck seat post puts a wrench in your plans but it isn't a deal breaker if you follow these tips to get it removed.
First, Try Lube
If you're lucky a seized seat post will come free after a good dousing with a lubricant like WD-40. Sometimes the post just needs a little kick start to get moving but more often than not this is more of a pipe dream. Another step you can try is to pour boiling water down the seat post opening in an effort to expand the metal sheath and allow the seat to slide out. More often than not you'll have to try the boiling water method a multitude of times before it (hopefully) works. Another effort you can take is to apply a chill zone product to the seat post to cool the metal to try and get it to contract.
The process of expanding and contracting the contacting metal of a stuck seat post will get you halfway there but the final removal is likely going to require some elbow grease. Work the seat back and forth with some effort to finally get it unseized. Once you've sweated through your t-shirt but have finally gotten the seat free, you don't want to emulate this workout again so apply a product called an anti-seize paste to the post to form a barrier that prevents getting stuck in the future.