learning spells / forgetting spells

[Xhanum]

I am curious about the process of learning spells, for those classes who must "memorize" spells (wizard, sorc, ?)
In short, once you memorize a spell is there a way to unmemorize it and learn something else? For example, I want to know lvl 2 spell "x", but as I grow in the world, I determine to use defensive rather than offensive spells, so I would rather use lvl 2 spell "y". If I choose a spell that I decide  not to use anymore, can I change it somehow, or is it simply part of the character for all subsequent levels once selected?

Thanks in advance for answering a noob question.

Pesar Fuladi
3/3 Ranger/Wizard

[IDii]

Wizards get to learn 2 spells every time they level and also get to read scrolls to add spells to their book. But they can't unlearn spells for others, so you have to just find a scroll or take them at level up if you want new spells, but the old ones remain in your book.

For sorcerers and bards it's different. You have a set amount of spells you know of each level. Like at lvl4 your sorcerer would know six cantrips, three level 1 spells and one lvl 2 spell... The next time you level to 5, you get like one more lvl1 and lvl2 spell and also get to change all your existing spells the way you want... can do this over and over as many times as you want but only on level ups. So you can change like all your spells if you want but between levels you're stuck.