To change your phone number, you need to deactivate the old number, and verify a new number.
Please go to your Truecaller settings > About Truecaller> Deactivate account.
Then you need to re-verify with the new SIM card (slot 1 if using dual SIM). You can only verify 1 number per Truecaller account.
For Android devices please tap on the Menu > Edit profile > Tap on your number and press 'Continue'. See below screenshots for references.