How can I disable mod_security in .htaccess file?

It is possible to do this, but most likely your host implemented mod_security for a reason. Be sure they approve of you disabling it for your own site.

That said, this should do it;

<IfModule mod_security.c>
  SecFilterEngine Off
  SecFilterScanPOST Off
</IfModule>

Just to update this question for mod_security 2.7.0+ - they turned off the ability to mitigate modsec via htaccess unless you compile it with the --enable-htaccess-config flag. Most hosts do not use this compiler option since it allows too lax security. Instead, vhosts in httpd.conf are your go-to option for controlling modsec.

Even if you do compile modsec with htaccess mitigation, there are less directives available. SecRuleEngine can no longer be used there for example. Here is a list that is available to use by default in htaccess if allowed (keep in mind a host may further limit this list with AllowOverride):

    - SecAction
    - SecRule

    - SecRuleRemoveByMsg
    - SecRuleRemoveByTag
    - SecRuleRemoveById

    - SecRuleUpdateActionById
    - SecRuleUpdateTargetById
    - SecRuleUpdateTargetByTag
    - SecRuleUpdateTargetByMsg

More info on the official modsec wiki

As an additional note for 2.x users: the IfModule should now look for mod_security2.c instead of the older mod_security.c

On some servers and web hosts, it's possible to disable ModSecurity via .htaccess, but be aware that you can only switch it on or off, you can't disable individual rules.

But a good practice that still keeps your site secure is to disable it only on specific URLs, rather than your entire site. You can specify which URLs to match via the regex in the <If> statement below...

### DISABLE mod_security firewall
### Some rules are currently too strict and are blocking legitimate users
### We only disable it for URLs that contain the regex below
### The regex below should be placed between "m#" and "#" 
### (this syntax is required when the string contains forward slashes)
<IfModule mod_security.c>
  <If "%{REQUEST_URI} =~ m#/admin/#">
    SecFilterEngine Off
    SecFilterScanPOST Off
  </If>
</IfModule>