Tutorial Video. Step by step below;

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

Select all the files by clicking the Checkbox near /home/container/ and click the Delete button.

6

Sixth Step

Under the Minecraft Section, Go to the Version tab and search for the version you want to install.

7

Seventh Step

Click on the Install button

8

Eighth Step

Start your Server afterward.

9

Ninth Step

Accept the EULA by pressing the I Accept button

There you go! You just reset your entire server to your preferred server software!
How to add a Server IconHow to install/add plugins