How to Redirect huge numbers of URLs to another URLs?

If /seba-online-form-fill-up-2018.html is an actual WordPress URL then this is relatively trivial to do in .htaccess. For example, the following one-liner using mod_rewrite could be used. This should be placed before the existing WordPress directives in .htaccess:

RewriteRule ^\d{4}/\d{1,2}/(.+\.html)$ /$1 [R=302,L]

This redirects a URL of the form /NNNN/NN/<anything>.html to /<anything>.html. Where N is a digit 0-9 and the month (NN) can be either 1 or 2 digits. If your Blogger URLs always have a 2 digit month, then change \d{1,2} to \d\d.

The $1 in the substitution is a backreference to the captured group in the RewriteRule pattern. ie. (.+\.html).

Note that this is a 302 (temporary) redirect. You should change this to 301 (permanent) only when you have confirmed this is working OK. (301s are cached hard by the browser so can make testing problematic.)

Well, there are some Data base migrator plugins for that porpuse like Migrate DB, obviuosly this is for changes inside a SQL data base used for wordpress and basically this plugin will look for the old URL to change it for the new URLS. So you can search and replace like:

Search: http://yoursite.in/2017/10/
replace: http://yoursite.in/

you will have http://yoursite.in/xxxxxx.html

Also you can do the same with a text editor like NOTEPAD++ in your backup file. Both method works