We can't modify public repositories from other companies for obvious reasons.

But you can download, modify and apply it!

Using your bitnami/tomcat as example.

• On Helm 2 you can use fetch:

$ helm version
Client: &version.Version{SemVer:"v2.16.1", GitCommit:"bbdfe5e7803a12bbdf97e94cd847859890cf4050", GitTreeState:"clean"}

$ helm repo add bitnami https://charts.bitnami.com/bitnami
"bitnami" has been added to your repositories

$ helm fetch bitnami/tomcat

❯ ls
tomcat-6.2.4.tgz

• If you are running Helm 3 the fetch was replaced by pull:

$ helm version
version.BuildInfo{Version:"v3.0.2", GitCommit:"19e47ee3283ae98139d98460de796c1be1e3975f", GitTreeState:"clean", GoVersion:"go1.13.5"}

$ helm repo add bitnami https://charts.bitnami.com/bitnami 
"bitnami" has been added to your repositories

$ helm pull bitnami/tomcat                                

$ ls
tomcat-6.2.4.tgz

• It will download a tgz of the chart, just unpack it, modify what you want carefully and then you can apply it locally pointing to the folder where it was unpacked:

$ tar -xvzf tomcat-6.2.4.tgz 
tomcat/Chart.yaml
tomcat/values.yaml
tomcat/templates/NOTES.txt
tomcat/templates/_helpers.tpl
tomcat/templates/deployment.yaml
tomcat/templates/ingress.yaml
tomcat/templates/pvc.yaml
tomcat/templates/secrets.yaml
tomcat/templates/svc.yaml
tomcat/.helmignore
tomcat/README.md
tomcat/ci/values-with-ingress-and-initcontainers.yaml

$ ls
tomcat  tomcat-6.2.4.tgz

$ cd tomcat 

$ ls
Chart.yaml  ci  README.md  templates  values.yaml

$ head Chart.yaml 
apiVersion: v1
appVersion: 9.0.31
description: Chart for Apache Tomcat
home: http://tomcat.apache.org
icon: https://bitnami.com/assets/stacks/tomcat/img/tomcat-stack-110x117.png
keywords:
- tomcat
- java
- http
- web

$ helm install . --generate-name
NAME: chart-1583237097
LAST DEPLOYED: Tue Mar  3 13:04:58 2020
NAMESPACE: default
STATUS: deployed
REVISION: 1
TEST SUITE: None
NOTES:
** Please be patient while the chart is being deployed **
...
$ helm3 list
NAME                    NAMESPACE       REVISION        UPDATED                                 STATUS          CHART           APP VERSION
chart-1583237097        default         1               2020-03-03 13:04:58.617410239 +0100 CET deployed        tomcat-6.2.4    9.0.31     

• I didn't changed anything but as you could see the chart is open for you to modify as you like.

• You can even create a private repository for your custom charts, learn more here: The Chart Repository Guide