If you have already filed your return, you cannot change your Pension Income Split amount using any online CRA services, such as ReFile or Change My Return (using your CRA My Account).
You must complete T1 Adjustments for both you and your spouse and mail the adjustments to your tax centre.
https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/change-your-return.html