when uploaded custom pages(embed <%instruction%> tags),how can i edit this custom text just like the
default standard page in central manager?
If you are using CSP (Custom SignIn Pages) then you must edit the files in the archive and upload it again.
There's a nice manual for CSP.
If your just looking to quick set text quickly in Custom Sign-In pages add the following at the top of the file:
<% SET portal = 'You Portal' %>
You can also do other things like changing the back group colour or setting the help button on/off:
<% SET color ='#006666' %>
<% SET help_on = 0 %>
Check out the Custom Sign-In page guide for a full list of variables.