1

First Step

2

Second Step

Select your Server
3

Third Step

Make sure to Turn OFF the Server
4

Fourth Step

Under the General Section, Click the Files tab
5

Fifth Step

Click the server.properties file
6

Sixth Step

Scroll down to till you see max-players: [value] and edit the [value] to your prefered number and hit the Save Content button
7

Seventh Step

Now, Head to the Console tab and hit Start
There you go, You have changed the maximum capacity of players able to join your Server!