There are different ways to approach this. One is to get the opposing counsel to stipulate to permit you to amend the demurrer. Another way is to take your demurrer off calendar and refile the new demurrer. Without knowing the groiund or grounds for your original demurrer, it is difficult to advise you further.
The $20 difference in the rent being claimed does matter in a residential lease. The 3-day day notice to pay rent or quit must clearly state the exact amount of rent due must be stated clearly on the notice. If the amount is overstated, the notice is fatally defective. This defect is normally raised in the answer and proven at the time of trial, rather than in a demurrer.
You can withdraw and refile the Demurrer. The amount of rent does matter in a residential case, so be sure to raise that issue. A demurrer, however, may not be the best place to raise these issues because they require evidence. Whenever you have a hearing with the judge, be sure to raise these issues there too, he should accept it from you as a pro se defendant, but I would really recommend getting a residential landlord/tenant lawyer to help you with this. I don't handle this type of thing (only commercial real estate), but if you message me I can provide some referrals.