You should be able to change your 401(k) contributions at any time throughout the year. There's no open enrollment period for this. You may have to wait until the next full pay period for changes to take effect, but you shouldn't have to wait 3-6 months.
For an IRA, if you have a 401(k) at work you need your MAGI to be below $60k (as a single person) to make those contributions be fully deductible. The deduction gets phased out if your MAGI is between $60k and $70k. If you have pre-tax deductions (401(k), health insurance, HSA, etc.) that bring your MAGI down below $70k, you could potentially make some amount of deductible IRA contributions. The more you can put in your 401(k), the higher your IRA contribution limit will be.