Unlike TS3's Lifetime Wishes, you may change your Aspiration at any time by pressing 'G' and going to the menu. The Sims 4's Aspirations are life goals that Sims may select, allowing them to complete a checklist of objectives to earn Satisfaction that may be spent toward Rewards.
The Sims 4 Aspirations List Reward Traits, Satisfaction Totals, and Milestone Summaries