Please don't do that!
You should never use string concatenations in your sql queries. Always use parameterized queries. This kind of string concatenations are open for SQL Injection attacks.
With this concatenations, you might forget to use some comma, quotes, brackets etc..
Also use the using statement to dispose your Connection and Command. For example;
using(OleDbConnection con = new OleDbConnection(ConnectionString))
using(OleDbCommand cmd = com.CreateCommand())
{
string s = "UPDATE tableAbsensi SET Absen_keluar=? WHERE ID=? AND Tanggal=?";
cmd.CommandText = s;
cmd.Parameters.AddWithValue("@absen", DateTime.Now.ToString("hh:mm"));
cmd.Parameters.AddWithValue("@id", idkaryawantxt.Text.ToString());
cmd.Parameters.AddWithValue("@tanggal", DateTime.Now.ToString("MM-dd-yyyy"));
cmd.ExecuteNonQuery();
}