I have an accounting firm and I am planning to start using Manager for myself and my customers.
I have some issues regarding the accounting system and I would like to have a call with the managers of Manager so we can address those issues and possibly fix them for my version at least.
There is no customization support for Manager. Read the Guides. Practice with test businesses. Ask questions on the forum. If you think there are things that need to be fixed, the forum is the place to raise issues.
In addition to what was advised by @MCasper and @Tut the basics of Manager involve the different products: Desktop, Cloud or Server.
If you want your Customers to use Manager for free on their Desktop without user login and user permissions then point them to Download - Manager.io and download, install and use the appropriate Desktop edition for their system (Window, Linux or Apple). These are single user versions for which the usage details and frequently asked questions are provided als at Download - Manager.io below the links.
If you want to host and manage the accounting businesses for your customers then the Cloud (fully hosted and maintained by Manager) is the preferred option if you do not want headaches and if you have limited IT skills to manage a Server. The Cloud product is also cheaper than the Server product (install on your own or VPS or Dedicated Windows, Linux (Ubuntu) or Apple Server) on a month and yearly basis because you need to renew maintenance license of the Server Edition annually.
Comparison between 1 & 2 above is at Cloud Edition - Manager.io. The main differences compared to the Desktop product are: a) Paid license, b) Multi-User Access, c) User Permissions, and because of web-interface accessible locally and remotely ons desktops, laptops, tablets, phones, etc. anything with a web-browser.
All Products use the same version and have the same accounting functions and allow you to create unlimited businesses (limits for desktop and server products depending on your hardware).
Could you please elaborate with a comparison between the server product and the cloud product? And how would I go about the updates when using the server product.
There is nothing to compare except that the Cloud edition is hosted by Manager and the Sever edition is hosted by yourself either on an onsite server, a Virtual Private Server (VPS) or Dedicated Server. Shared hosting servers will not work.
Updating the Server product is done by yourself running terminal scripts on the server in which you installed Manager Server. Be aware that there is no support for managing and running servers, you need in-house IT expertise.