In order to do this you will need to check the Play in Commissioner Mode box in your Manager Options tab:
Now navigate to your team Front Office tab and you will see a button on the right hand side saying Edit Finances:
Once you have clicked on that you will see on the left-hand side of the next screen (and scrolling down to the get to the part I have shared below) you have numerous options from merely boosting your actual budget, which would be a short-term fix, to bumping up things like Fan Interest, Fan Loyalty, and Market Size, which would provide more long-term effects but which I can't promise will help in the short-run.
I can't speak to any possible unintended consequences in altering these and I would suggest not making drastic changes and possibly just boosting things gradually over time. The only time I ever messed with these was when I was first playing the game (so several versions ago) and I took over the Colorado Rockies in their inaugural season and a few seasons in I boosted the Fan Interest rating because I felt that with the team moving into Coors Field, and just with how crazy good attendance numbers were for the early Rockies teams especially at Mile High, the game was unfairly keeping attendance numbers depressed for the team. Having done so, the budget did start to go up before long and that may have not been terribly realistic either.