Hi Toni and welcome to the forum.
The first rule is to make sure you have a safe backup copy available and that you are not interfering with any live operations!
After that ... it depends on what you need to do to update the existing models.
Also, perhaps, how complex they are compared to what you have learned about Monarch so far.
If you are moving from one version of Monarch to a newer one Monarch will usually do the work for you.
If you need to make some fairly simple amendments just open the model, make the changes and save it (but not my opening comments above!)
If major surgery is required and you have a lot of models working together in the complex process ... it's still the same really but you may need to plan your activity quite carefully and be sure you understand the overall process.
If you have a bunch of new requirements ... it may be worth considering starting afresh.
The best thing I can suggest is to provide some examples to the community (if you are able to within data protection rules) and see what feedback you get. We can then take things from there.
Help can be provided offline if required fir security reasons.