Increment selected textbox (read only) using buttons in WPF

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WPF

MVVM

SelectedValue

I have a grid inside a stack panel.

On the grid I will place several different textboxes that will show positional values that the boxes are databound from my database.

I will want to have buttons to increment/decrement by small, medium, large, etc. amounts.

This is a touchscreen display so I want the user to touch the textbox (or whatever control I should use) and then with it "selected" I want the user to be able to touch one of the several increment/decrement buttons.

The increment/decrement buttons using relay commands need to take the value of the selected textbox(or what control I should use) and then increment/decrement it.

As a note, I will need to be able to know whether and what has changed so I can allow the user to save the changes if they want. I'll need to be able to know what was changed so I can update the database with the new values.

I'm using MVVM with WPF and am stuck as I'm new to WPF.

What I have tried:

Below is my RelayCommand. There are a couple things I've tried and commented out because they didn't work. I did hard code a value of one of the textboxes I created just to make sure, as a start, that I could increment by +5 and it would update properly. This is working. I have no idea how to "pass" that property/value to the RelayCommand.

public RelayCommand cmdIncrementSmall { get; private set; }
public void doIncrementSmall(object? o)
{
    // SelectedAssemblyPosition = item as short;
    //IInputElement focusedElement = FocusManager.GetFocusedElement;
    _LeverHeightLeft += 5;
}

Below is the StackPanel that I've created with the textbox and button as a start to begin testing. Once I have it figured out I can add the other textboxes (or whatever control I should use) and the other increment/decrement buttons.

<stackpanel x:name="ExercisePositionSetter" margin="1255,3,0,0" height="2017" background="Teal">
<grid>
    <grid.columndefinitions>
        <columndefinition width="160">
        <columndefinition width="160">
        <columndefinition width="160">
        <columndefinition width="160">
        <columndefinition width="160">
        <columndefinition width="160">
        <columndefinition width="160">
        <columndefinition width="160">
        <columndefinition width="160">
        <columndefinition width="160">
        <columndefinition width="160">
        <columndefinition width="160">
        <columndefinition width="160">
        <columndefinition width="*">
    

    <grid.rowdefinitions>
        <rowdefinition height="125">
        <rowdefinition height="125">
        <rowdefinition height="125">
        <rowdefinition height="125">
        <rowdefinition height="125">
        <rowdefinition height="125">
        <rowdefinition height="125">
        <rowdefinition height="125">
        <rowdefinition height="125">
        <rowdefinition height="125">
        <rowdefinition height="125">
        <rowdefinition height="125">
        <rowdefinition height="125">
        <rowdefinition height="125">
        <rowdefinition height="125">
        <rowdefinition height="135">

    
    
    <textblock grid.column="0" grid.row="1" horizontalalignment="Center" verticalalignment="Bottom" fontsize="42" fontweight="SemiBold" textalignment="Center">
        Lever <linebreak>
        Height
    
    <textbox x:name="LeverHeightLeft" grid.column="0" grid.row="2" horizontalalignment="Center" verticalalignment="Bottom" fontsize="42" fontweight="SemiBold" textalignment="Center" 
text="{Binding Path =_LeverHeightLeft}" width="125" isreadonly="True">