how to add prefix json string in golang

I am newbie. I have a image in my db which is stored as a path to it. In api json I have to output this path and add a prefix in front of it, how can I do that?
My function:

func GetAllSliders(c *gin.Context) {
	var sliders models.Slider

	config.DB.Model(models.Slider{}).Where("id=?", 1).Update("image", ("https://spares-dt.ru/" + models.Slider{}.Image)) //i tried this, but it doesnt work

	if err := config.DB.Find(&sliders).Error; err != nil {
		c.JSON(http.StatusInternalServerError, err.Error())
	} else {
		c.JSON(http.StatusOK, gin.H{"data": sliders})
	}
}

Json output i have:

{
    "data": {
        "id": 1,
        "main_text": "123",
        "upper_text": "123",
        "down_text": "123",
        "image": "data/photos/sliders/image.PNG"
    }
}

I want:

{
    "data": {
        "id": 1,
        "main_text": "123",
        "upper_text": "123",
        "down_text": "123",
        "image": "https://spakes-dt.ru/data/photos/sliders/image.PNG"
    }
}

And my struct:

type Slider struct {
	Id        uint   `json:"id" gorm:"primaryKey"`
	MainText  string `json:"main_text"`
	UpperText string `json:"upper_text"`
	DownText  string `json:"down_text"`
	Image     string `json:"image"`
}

Simply change the Image field's value before JSON marshaling it:

if err := config.DB.Find(&sliders).Error; err != nil {
    c.JSON(http.StatusInternalServerError, err.Error())
} else {
    sliders.Image = "https://spares-dt.ru/" + sliders.Image
    c.JSON(http.StatusOK, gin.H{"data": sliders})
}

This is an example of how custom json marshalling works. Modifying your model by adding the MarshalJson method will help you to customize.

package main

import (
	"encoding/json"
	"fmt"
)

type Slider struct {
	ID        int    `json:"id"`
	MainText  string `json:"main_text"`
	UpperText string `json:"upper_text"`
	DownText  string `json:"down_text"`
	Image     string `json:"image"`
}

//Add MarshalJson function
func (u *Slider) MarshalJSON() ([]byte, error) {
	return json.Marshal(&struct {
		ID        int    `json:"id"`
		MainText  string `json:"main_text"`
		UpperText string `json:"upper_text"`
		DownText  string `json:"down_text"`
		Image     string `json:"image"`
	}{
		ID:        u.ID,
		MainText:  u.MainText,
		UpperText: u.UpperText,
		DownText:  u.DownText,
		Image:     "https://spakes-dt.ru/" + u.Image,
	})
}

func main() {
	data := Slider{Image: "data/photos/sliders/image.PNG"}
	bdata, _ := json.Marshal(data)
	fmt.Println(string(bdata))
}

This will produce the following output.

{
  "id": 0,
  "main_text": "",
  "upper_text": "",
  "down_text": "",
  "image": "data/photos/sliders/image.PNG"
}

Update your code something like this after you create MarshalJson method

func GetAllSliders(c *gin.Context) {
    var sliders models.Slider

    config.DB.Model(models.Slider{}).Where("id=?", 1)//i tried this, but it doesnt work

    if err := config.DB.Find(&sliders).Error; err != nil {
        c.JSON(http.StatusInternalServerError, err.Error())
    } else {
        b, _  := json.Marshal(sliders)
        c.JSON(http.StatusOK, gin.H{"data": string(b)})
    }
}

you should use exec

config.DB.Exec(`UPDATE slider SET image = "https://spares-dt.ru/" + iamge WHERE id = ?`, 1)