On your dashboard, click on "Account Details" in the Portfolio Balance box.
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In the "Accounts" section, click on the dropdown that says "I want to..." and choose "change my contributions."
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Click on "Change Contributions."
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From here, you can see your current contribution amounts listed under "Current Contribution Election." Type in the new contribution amounts you wish to change to, for either the tax sheltered Traditional or Roth plans (you can contribute to one or both of these- see Traditional vs. Roth Contributions)
IMPORTANT: If you are trying toswitch your contributions from a Tax Sheltered plan to a Roth (or vice versa), make sure to not only list the new contribution amount next to the account type that you want to contribute to, but to ALSO list $0 (as the "new contribution" amount for the account type that you want to stop contributing to. Employees have the option to contribute toward both types of accounts, so if you do not zero one of them out, then the system will assume you want to continue with the prior contribution amount for the other type of contribution, and add the new one on top as a second contribution. Double check the amount listed for the overall "Total" so you can confirm that is the amount you want deducted from your paycheck.
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Mark the checkbox to indicate you "agree," and the click "Next."
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On the next screen, review the change to make sure it is correct and click "Confirm."