Change label text in ascx through JS function in aspx

See more:

C#

Javascript

jQuery

ASP.NET4

I am new to Javascript and kinda struck at one point. So here is what I want to achieve:

I have a user control (having a textbox and label)

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="JMTextBox.ascx.cs" Inherits="Controls_JMTextBox" %>

<asp:Panel ID="pnlMain" runat="server" BackColor="White">
			<table style="cellPadding:0px; width:100%;">
				<tr>
					<td>
						<asp:TextBox runat="server" ID="txtEntry" CssClass="form-text required" />
					</td>
					<td>
						<div class="box" id="dvAlert">
							<div class="center">
								<div class="dialog-1">
									<asp:Label runat="server" ID="lblAlertText" style="padding:5px;" Width="200" ></asp:Label>
									<div class="left-point"></div>
								</div>
							</div>
		
						</div>
					</td>
				</tr>
			</table>
		</asp:Panel>

JMTextBox.js:

function SetLabelText(textStr) {
	<<change label text here>>
			}

In the code behind, I have 2 string properties: AlertText and InfoText

public string InfoText
	{
		get { return _infoText; }
		set { _infoText = value; }
	}

	public string AlertText
	{
		get { return _alertText; }
		set { _alertText = value; }
	}

On aspx, I am using this control in aspx page in an entry form

<JM:JMTextBox ID="txtUserName1" runat="server" class="required" InfoText="This is Info" AlertText="This is alert" />

By default, label displays info text, but in JS function of aspx page, where fields are validated, I want to change info text to alert text.

Thanks in advance

What I have tried:

I have tried to google it out but mostly getting answers to invoke JS function from code behind using

Page.ClientScript.RegisterStartupScript

which isn't my requirement. I want to change it through JS only when fields are validated.