Box 20 - Taxable CPP Benefits: T4A(P) does not give me option to change this number to new 2019 figures which is $13,803.36 what is benefit number which is optional?

You should be able to report the maximal retirement benefit of 2019 in Box 14 of the T4(A) slip.

Box 20 is the total amount of all benefits paid under the Canada Pension Plan (retirement + disability + post-retirement + survivor, etc). However, you still have to break them down under the appropriate boxes as reported on the bottom right side of your T4A(P) slip.

 

Please check the screenshot below for reference:

 

I hope this was helpful