Thinking on How to Bulk Update Template field values

Template values are stored as an encoded query, you can do mass update using script. Please refer below script as reference:

Sample script :

var grr= new GlideRecord('sys_template');
grr.query();
while(grr.next())
{
grr.template += '^active=true';
grr.update();
}

Advance Script :

var encodedString = '^u_template_applied_by=javascript:gs.getUserID()^u_template_used=';
var grTem = new GlideRecord('sys_template');
//grTem.addQuery('sys_id', '692c6a13dbd4ef0048f171efbf961983'); // Add more query lines to restrict just to templates that need updating
grTem.addQuery('global', true).addOrCondition('group', '!=', '');
grTem.query();
gs.print(grTem.getRowCount()); // Print out the number of records being updated
while (grTem._next()) {
gs.print(grTem.template);
    grTem.template = grTem.template.replace('^EQ', '') + encodedString + grTem.name.toString() + '^EQ';
    grTem.setWorkflow(false); // Don't run business rules, etc.
    grTem.autoSysFields(false); // Don't update system fields
    //grTem.update(); // Un-comment this line once you're sure the row count is just the records you want to update.
}