A Sketch document has structures similar to any document consisting of layers, so if you have ever opened Adobe Photoshop file or Adobe XD it will not be difficult for you to figure it out. The template file is built on the basis of both Simple Layers and Layers called Sketch Symbols.
Some templates require libraries to be enabled. Better to do it right away. How to do this is written in the article about Adding Sketch Libraries
Customization of any element consists of two steps – the Сhoice of Element / Layer and its change. To select a layer, you need to use the Layers Panel on the left side of the Sketch window, and for setting up the Inspector Panel on the right side of the Sketch window.
You can learn more about the Sketch interface from The Interface section of the official documentation.
Each template contains at least two pages, the Symbols page contain all Document Symbols that are not included in the library are located, as well as the Layouts Page(s), which contains the Template Design Layout. There may be several pages with Layouts.
Each page contains one or more Artboards. Artboard is a layout of a web page or a window of the UI.
Any of artboards contains several layers. In the Layers List, there are both Simple Layers(as Text Layer, Path Layer etc.) and Symbol Layers, which also consist of simple layers.
To Collapse/Expand the Directory with layers or Artboard, click on the arrow icon to the left of the name.
To Select a layer, click on its name once.
To Rename a layer, click on its name one more time.
To Hide/Show a layer, click on the icon with a crossed eye to the right of the layer name. Hold the ⌥ O
To start editing text in a text layer. Select a text layer and press T after that put the cursor in the right place in the text and start typing.
To open a Symbol for editing, select it and press ⏎ Enter
You can learn more about Pages, Artboards, and Layers from the Layers List of Official Documentation.
To change the style of elements, such as color or font, use the Inspector panel. Learn all about how to change the layer style from the Inspector section in the official Sketch documentation.
To change the Background Color, Border or change the Background Image, use the style panel. Click on the color box to select the new color for the item.
You can also change the color of the element with the help of predefined styles. In the Appearance panel, select a different style for the element. Usually, color styles are in the Palette heading inside Document styles.
In the same way, you can change the style of an element inside a Symbol. To do this, select the required object inside the symbol and change its style with the drop-down list.
Adding and replacing Symbols
You can easily replace a Symbol with another one whose sizes coincide. To do this, use the drop-down list of symbols in the Inspector panel.
Also, you can add a symbol from the symbol menu. Go to menu Insert / Symbols / Document, select a symbol and place it on the page.