Go to about:manual → Bridges or
Go to https://tb-manual.torproject.org/ → Bridges

Read the instructions.

Tip: If you are in a location where you can connect to Tor, get a new obfs4 bridge address at https://bridges.torproject.org/options → Do you need a Pluggable Transport? obfs4 → Get Bridges → complete the Captcha challenge → copy a bridge address (copy the whole single address), which starts with obfs4 and ends with either mode=0, mode=1, or mode=2

Follow the User Manual instructions: Go to Tor Browser > Preferences > Connection > Bridges > Add a New Bridge > Enter a bridge address you already know > Add a Bridge Manually > paste an obfs4 bridge address in the Provide Bridge field > OK > restart Tor Browser. When it restarts, it should use the bridge to connect to the Tor network.

See https://support.torproject.org/ > Glossary > obfs4.

Tip about typing about:*** in the URL address bar: Go to about:about, which lists all available about:*** pages.