See: https://upgrade.odoo.com/module/upload

You can probably upgrade your custom modules yourself very easily if you are a little bit technical.
There are some cases where upgrading your modules is relatively easy:

• if your custom modules defines custom models (models not related to any standard models)
• if your custom modules just adds new and simple (Char, Float, Integer, ...) custom fields
• if your custom modules adds custom fields linked to a standard model (and this standard model has not changed bewteen versions)

The difficult parts are mostly:

• views inheriting from standard views that have changed in the new version
• models inheriting from standard models that have disappeared or changed heavily
• fields related to standard models that have disappeared or changed heavily
• methods that were using models that have changed or disappeared

All currently released (supported) and unreleased (unsupported) versions of Odoo are available from the same GITHUB repository.  The SAAS version is updated several times a year.  The 'next' version of Odoo is called MASTER in the list of branches.

You might also benefit from reading more about the upgrade service offered for Enterprise databases - https://upgrade.odoo.com/database/upload

The SAAS version isn't meant to be installed on your own server - and you should check if it is eligible for support and upgrades if you do so.