A colloquial expression meaning 'if that's the case / if you're going to do that, then ...', used to suggest taking advantage of the situation or making a certain choice.
どうせなら最後までやってみよう。
If we're going to do it anyway, let's try it until the end.
どうせなら新しい店に行ってみない？
If we're going to go out anyway, shall we try that new restaurant?
時間があるなら、どうせなら練習しておいたほうがいい。
If you have time, you might as well practice.
もう直すつもりなら、どうせなら全部直してしまおう。
If you're going to fix it anyway, you might as well fix everything.