If you have a website that has a content management system attached to it, then you will need to receive training on how to use it before you can start making changes to the site. It is important to ensure you understand exactly what you are doing before you make changes especially if the site is already live as it could cause issue with certain elements not working or even take the site offline completely if you make a mistake.
Some web developers use off the shelve content management systems such as WordPress and others use their own which they have built. There are advantages and disadvantages to both. If your website is very bespoke and you need a lot of functionality built in, then it may be better for your site to be built using a bespoke content management system. If it is fairly straight forward then an off the shelf one will do just fine.
When you receive your training make sure that you make nots on exactly what you need to do. You may even want to print off screen grabs or take photos on your phone if certain screens as you can then make notes on them for you to refer to later should you need to.