Download El Capitian

Given the complexities of update/upgrade scenarios be sure

to understand that results from a general search engine can

be too vague and show several false-starts.

In the example shown, avoid mackeeper, macpaw, etc since

these sites may be littered with add-ons, spam, &/or malware.

The Apple Support site to read and understand, directly shows

& tells of what you need; but could be unclear how to get it..

• How to get old versions of macOS - Apple Support

//support.apple.com/en-us/HT211683

Instructions vary, between newer Mac systems and older ones.

Details are in the Apple source article; study them, before use.

Read carefully, & use Safari browser to get correct downloadable

installers from the Apple Servers. Expect these to take time; and

then later on, even more time. Your Mac may start, then restart.

IF your Mac identifies as one that could use a newer macOS

than you suggest, or imply you want or need; the newer one

has better odds of supplying less outdated Browser versions.

[There's newer Firefox, Brave, & others to use for older macOS.

However some sites won't load content, in too old a browser.]