Nofollow links do not help you in getting a better SEO rank. People follow those links, but those links do not pass any authority to your website.
Even Google crawls that links with rel= "nofollow" tag, but it does not pass the page rank to the destined URL.

The rel= "nofollow" tag is used when the website does not want its authority to pass to the outbound link. So that you do not get penalized by Google because you are linked to some low-quality website. Or you added so much outbound links to your website.

Add Nofollow To Link is an excellent SEO tool for quickly changing all links to nofollow links.
Or select specific links to nofollow from HTML loaded.

If you have a website or you are related to the SEO profession. You must be well aware of domain authority, page authority, link juice, dofollow, and nofollow links.

Before going any further, let's define these terms.
Domain authority(DA) means how likely your domain is to rank on search engine result pages (SERPs) relative to other relevant websites of that specific niche or industry.
Page authority means your specific webpage's strength to rank in search results compared to other relevant webpages of that particular niche or industry.
Link Juice means the authority that other website passes to your domain or webpage.
Having a dofollow backlink means that another website is passing authority to your domain or webpage link and sharing its link juice to your domain or web page.
Having a nofollow backlink means that another website authorizes your domain or webpage link to be part of its website but does not share its link juice to your domain or webpage.

Always nofollow those links that are
- In the forums or comment section
- Sponsored links 
- Links in the press release

One way is to do it manually.
Add a rel=“nofollow” attribute to the  tag.
For example, you want to add the tag to the following hyperlink.
Outbound link
Add the rel= "nofollow" attribute in your hyperlink.
The above link will look like this, after the addition of the nofollow attribute
"nofollow" href="http://www.SponsorSite.com">Outbound link
Or you can use some online tools to do this.

• Open the Add nofollow to link tool.
• You can either copy-paste the HTML code or load from the URL, or use the "browse" option to upload it.
• After uploading the code, suppose you want to add the nofollow attribute to all your tags. Click on the button "Add nofollow to all the tags." 
• Suppose you want to select the specific tags from the uploaded HTML code. In which you want to add the nofollow attribute. Click on the button "Let me select the tags." A popup window will appear, having all the tags from the uploaded code. Select the tags in which you want to add the nofollow tag.
• The result will be shown in the Result section.
• Please copy the code to paste it into the desired destination.

• Use the command "Ctrl + U" or right-click on your browser window and go to "view page source."
• Check the links in the HTML code.
• Links with rel= "nofollow" attribute are nofollow links. Otherwise, the links are dofollow links.

User privacy is our top priority. We do not keep a copy of the HTML code that the user uploads for adding the nofollow attribute in links.