You Link To Their Website In Return
Link exchanges used to be a common and widespread practice in SEO as they were a mutually beneficial way to improve each other’s rankings. As the practice got widely abused, Google devalued the trust and authority that passes between reciprocal links, however, don’t disregard them altogether as there’s still some value in them, and even if the value of these types of links has reduced, they can still send targeted traffic to your site. If someone says that they’ll only link to your site if you link to theirs, you need to be sure about two things.
Firstly, that they have a good quality website, which isn’t involved in spam-like practices (either on-site or off-site). Secondly, that there’s some logical connection between your site and theirs, such as through topic and/or location. If, for example, you offer computer repairs in Manchester, and you’re linking to a florist in Bristol, then the link may look suspicious to Google, and is better off avoided. Also, when engaging in link exchanges, try to ensure that the link to your site isn’t on some kind of links/partners page that’s filled with 10+ links to other sites. Getting linked to from within a blog post on another site is much preferable.