If you do nothing, you will likely have more tax taken out than needed and get a larger refund, which is another way of saying you are giving an interest-free loan to the government.
The IRS also has a calculator to help with your W-4 but it is temporarily out of service.
https://www.irs.gov/individuals/tax-withholding-estimator
If filing new W-4s, be sure to review all 4 pages of worksheets and instructions, not just page 1 with your signature that you give to your employer. And you will want to do the calculations once, but submit new forms to both jobs based on those calculations.
