You have a few options:
Option #1 - Leave the money where it is
If your balance is over $5k - you should be able to leave the money in your former-employer's 401(k). The money will stay there and continue to be invested in the funds that you elect to invest in. You should at the very least be receiving quarterly statements for the account. Even better - you should have access to some type of an online account where you can transfer your investments, rebalance your account, conform to target, etc. If you do not have online account access than I'm sure you can still transfer investments and make trades via a paper form. Just reach out to the 401(k) TPA or Recordkeeper that administers your plan. Their contact info is on the quarterly statements you should be receiving.
Option #2 - Rollover the money into your current employer's 401(k) plan.
This is the option that I tend to recommend the most. Roll the money over into your current employer's 401(k) plan - this way all the money is in the same place and is invested in the funds that you elect. Let's say you wanted to transfer your investments to a new fund lineup. Right now - you have to fill out the paperwork or go through the online process twice (for both accounts). Moving the money to your current-employer's plan and having all the money in the same place eliminates this redundancy, and allows you to make one simple transfer of all your assets.
Option #3 - Roll the money from your former-employer's plan into an IRA.
This is a cool option, because now you have a new IRA with a new set of dollar limits. You can roll the money into a separate IRA - and contribute an additional $5,500 (or $6,500 if you are 50+ years of age). So this is cool because it gives you a chance to save even more for retirement. Many IRA companies give you a "sign on bonus" where if you rollover your former-employers 401(k)...they will give you a bonus (typically a few hundred bucks - but hey its free money!).
Other things to note:
Take a look at your plan document from your former-employer's 401(k) plan. Take a look at the fees. Compare the fees to your current-employer's plan. There could be a chance that the fees from your former-employer's plan are much higher than your current-employer. So this would just be yet another reason to move the money to your current-employer's plan.
Don't forget you most likely have a financial advisor that oversees your current-employer's 401(k) plan. This financial advisor also probably takes fees from your account. So use his services! You are probably already paying for it! Talk to your HR at your employer and ask who the investment advisor is. Call the advisor and set up an appointment to talk about your retirement and financial goals. Ask him for his advice - its always nice talking to someone with experience face to face.
Good luck with everything!