Please try the following:
.aspx
<input id="btnRepayment1" type="submit" value="Repayment" runat="server" onserverclick="btnRepayment1_Click" />
<asp:Button ID="btnRepayment2" runat="server" Text="Repayment" onclick="btnRepayment2_Click" />
.cs
protected void btnRepayment1_Click(object sender, EventArgs e)
{
btnRepayment1.Value = "Paid";
}
protected void btnRepayment2_Click(object sender, EventArgs e)
{
btnRepayment2.Text = "Paid";
}
If this would be really helpful to you then don't forgot to Vote and Make Answer as Accepted.