Help building a side navigation bar

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I am new to building websites but familar with html and css codes. I have an template built using css code. I want to make a side navigation bar for my website. I can make it using html code but if I were to make a change to it at a later date, I would have to change the html code in every single webpage I have for that site. I want to know if anyone knows of an easier way to do this. Like having a single side navigation bar file that if I edit a link there it is automatically applied to all of my pages. I was looking for trying to insert an html file in another html file or using javascript. Any help would be great.