Congrats on making the decision to save more for retirement!
You'll need to make the adjustment with your retirement plan provider. Some providers allow you to make the adjustment online, others require a signed form. Your HR department should be able to point you in the right direction if you're unsure who the plan provider is.
The answer to your question depends on your plan and your employer. It's possible with many plans to establish online access to your retirement account, where you will then be able to adjust your contributions. If that option isn't available, your human resources department should be able to help you.
Keep in mind that while it's generally a great idea to save more for retirement, you should consider your company retirement plan in light of other investments you have available. Many people make the mistake of funding their employer's retirement plan and ignore other possibilities. Most of my successfully retired clients have used a variety of retirement planning strategies and investment choices to craft their retirement plans.